Osisko Development Announces Optimized Feasibility Study for Permitted Cariboo Gold Project with C$943 Million After-Tax NPV5% and 22.1% IRR at US$2,400/oz Base Case Gold Price; at US$3,300/oz Spot Gold C$2.1 Billion After-Tax NPV5% and 38.0% IRR

2025-04-28

TSXV - TSX Venture Exchange · Uranium · Rare Earth Elements · Gold · Canada

(All dollar amounts are expressed in CAD dollars, unless stated otherwise)

HIGHLIGHTS 1

Robust returns with base case after-tax NPV 5% of $943 million, unlevered after-tax IRR of 22.1% and payback 2 of 2.8 years at $2,400/oz gold price assumption. Using spot gold price of $3,300/oz, NPV 5% improves to $2,066 million, IRR 38.0%, and payback 2 of 1.6 years Average annual production of ~190,000 ounces of gold over a 10-year mine life (202,000 ounces in the first 5 years) with first gold anticipated in H2 2027, assuming construction commences in Q3 2025, subject to progress on ongoing project financing discussions Average TCC of US$947/oz and AISC of US$1,157/oz over the LOM, placing the Cariboo Gold Project within the lower half of the global cost curve for gold mines 3 Average base case LOM annual FCF of $158 million ($296 million per year in the first 5 years) Improved single-phase build over 24 months and direct ramp-up to 4,900 tpd with total initial capital cost of $881 million and sustaining capital of $525 million over the LOM Streamlined processing facilities into a single location and improved flowsheet design with incorporation of a gravity circuit and production of higher-grade concentrate product Strong support for local employment with up to 613 direct jobs created during peak construction and 525 permanent jobs during operations Significant opportunities to potentially enhance Project economics and extend mine life through conversion of Mineral Resources adjacent to Mineral Reserves through infill drilling

MONTREAL, Québec, April 28, 2025 (GLOBE NEWSWIRE) -- Osisko Development Corp. (NYSE: ODV, TSXV: ODV) (" Osisko Development " or the " Company ") is pleased to announce the results of a positive optimized Feasibility Study (" 2025 FS ") for its permitted, 100%-owned Cariboo Gold Project (" Cariboo Gold " or the " Project "), located in central British Columbia (" BC "), Canada. The 2025 FS was completed by BBA Engineering Ltd. (" BBA ") as lead independent consultant, and supported by other independent engineering firms, in accordance with National Instrument 43-101 – _Standards of Disclosure for Mineral Projects _(" NI 43-101 "). The Company intends to file the technical report in respect of the 2025 FS (the " Technical Report ") on SEDAR+ (www.sedarplus.ca) and on EDGAR (www.sec.gov) under Osisko Development's issuer profile within 45 days of the date of this news release. The 2025 FS confirms strong economics for a low-impact underground operation using mechanized bulk mining methods, with attractive operating costs, manageable capital requirements, and well-positioned to benefit from favorable macroeconomic and gold price trends. The process facilities have been designed to accommodate potential future throughput expansions.

Sean Roosen, Founder, Chairman and CEO, commented , "The completion of this optimized feasibility study represents a critical milestone for the Cariboo Gold Project, one of the few undeveloped, permitted gold projects in a Tier-1 jurisdiction 4 . The results reaffirm our view that Cariboo is a high-quality asset with robust returns and significant upside potential within the existing mine plan. Our immediate focus remains on advancing project financing and further de-risking the project toward FID, but, we believe additional work could support potential future production increases within the planned mine footprint. Additionally, our extensive land position around the Project area offers numerous opportunities for new discoveries in this prolific gold belt. With today’s favorable gold price backdrop and positive outlook, we believe this project is well-positioned to deliver substantial value to all stakeholders. We look forward to sharing further updates in the coming months."

A formal positive final investment decision, along with securing of a project financing package in the coming months would enable certain construction activities to commence in the second half of 2025, with project completion targeted for the end of 2027.

Osisko Development will host a conference call and webinar presentation by management on the 2025 FS results on Monday, April 28, 2025 at 11:00 a.m. ET , followed by a question & answer session. Details for dial-in, webcast access, and replay archive are available at the end of this news release.

OPTIMIZED FEASIBILITY STUDY OVERVIEW

The Cariboo Gold Project is envisioned as a traditional underground operation, employing mechanized long-hole open stoping to extract ore from gold-bearing vein corridors—an intricate network of mineralized quartz veins predominantly hosted within unmineralized sandstone. An improved flowsheet from the 2023 FS (as defined herein 5 ), supported by additional metallurgical testwork, envisions ore beneficiation to be exclusively completed at the Mine Site Complex. This would produce saleable gold doré from a gravity concentrate and ~66 tpd of high-grade flotation concentrate averaging ~133 g/t Au. The flotation concentrate would be transported by truck to the Port of Vancouver for transport and sale to a smelting partner. 2025 FS key summary results and assumptions are outlined in Table 1:

| Table 1: Cariboo Gold 2025 FS – Key Results and Assumptions (after-tax) | | | | | Metric | units | Base Case | Spot Case | | Gold price | US$/oz | $2,400 | $3,300 | | Exchange rate | USDCAD | 1.35 | 1.40 | | Net Present Value at 5% discount | $ mm | 943 | 2,066 | | Internal Rate of Return (IRR) | % | 22.1% | 38.0% | | Payback, from commercial production | years | 2.8 | 1.6 | | Average annual free cash flow 1 | $ mm | 158 | 314 | | Average AISC, LOM 1 | US$/oz | 1,157 | 1,167 |

1. All-in sustaining costs per ounce and free cash flow are non-IFRS measures or ratios. Refer to "Non-IFRS Financial Measures" at the end of this news release for more information. 2. Spot case is based on the LBMA gold price as of the close of business on April 23, 2025, rounded to nearest $100/oz and the USDCAD exchange rate is based on the Bank of Canada daily exchange rate, rounded to nearest five cents.

Key Improvements and Optimizations vs. 2023 FS

The 2025 FS incorporates several important improvements and de-risking initiatives over the 2023 Feasibility Study that better position the Project from an execution, financing, and operational perspective. Notable changes include:

Accelerated Development Sequence: Single-phase construction and ramp up directly to nameplate capacity of 4,900 tpd, which increases the LOM average gold production profile by 16% to 190,000 oz per year, and 202,000 oz per year in the first five years. Streamlined Processing: A single milling facility at the mine site removes the need (as had been previously contemplated) to transport flotation concentrate 116 km to the QR Mill. This reduces capital and operating costs by consolidating operations into one location. Improved Flowsheet Design: Updated metallurgical studies and testing has resulted in the addition of a gravity circuit which, combined with a rougher and cleaner flotation circuit, resulted in overall project gold recovery of 92.6% and the production of ~66 tpd higher-grade concentrate product (reduction from 590 tpd in 2023 FS Phase II) averaging ~133 g/t Au (vs. 28 g/t Au in 2023 FS Phase II). Approximately 46% of gold is expected to be recovered by gravity. Underground Mine Design: Increased average stope size by ~60% compared to the 2023 FS, significantly reducing the total number of stopes required to achieve average daily throughput. Optimization of the geotechnical design of the mined stopes, supported by recent trial mining, allows for more operational flexibility of underground operations. Mineral Reserves: Probable Mineral Reserves remained largely unchanged, increasing slightly to 2.071 million ounces Au (17.8 Mt grading 3.62 g/t Au). Permitted: Project design and sequencing contemplated in the 2025 FS is aligned with the BC _Mines Act _and _Environmental Management Act _permits obtained in Q4 2024.

| | | | | | Table 2: Cariboo Gold 2025 FS vs. 2023 FS – Project Operating and Financial Metrics | | | | | Assumptions | units | 2023 FS | 2025 FS | | Gold price | US$/oz | 1,700 | 2,400 | | Exchange rate | USDCAD | 1.30 | 1.35 | | Discount rate | % | 5.0% | 5.0% | | Production | | | | | Mine life | yrs | 12.0 | 10.0 | | Total ore mined | kt | 16,703 | 17,815 | | Peak annual throughput | tpd | 4,900 | 4,900 | | Average gold head grade | g/t Au | 3.78 | 3.62 | | Total contained gold | koz | 2,031 | 2,071 | | Avg. gold recovery | % | 92.0% | 92.6% | | Total recovered gold, payable | koz | 1,869 | 1,894 | | Avg. gold production, LOM | koz/yr | 164 | 190 | | Avg. gold production, first 5 yrs | koz/yr | 96 | 202 | | Operating Unit Costs | | | | | Underground mining | $/t mined | 53.6 | 62.3 | | Processing | $/t mined | 31.2 | 23.2 | | Water and waste management | $/t mined | 7.2 | 5.0 | | Electrical transmission line | $/t mined | — | 4.9 | | General and administrative | $/t mined | 10.7 | 15.4 | | Total unit operating costs | $/t mined | 102.6 | 110.7 | | Total operating costs | $ mm | 1,714 | 1,921 | | Royalty payments | $ mm | 206 | 292 | | Offsite charges | $ mm | 5 | 143 | | Operating Costs | | | | | Total cash costs 2 | US$/oz | $792 | $947 | | AISC 2 | US$/oz | $968 | $1,157 | | Capital Expenditures | | | | | Initial costs | $ mm | 137.3 | 881 | | Expansion costs | $ mm | 451.1 | — | | Sustaining costs | $ mm | 466.6 | 426 | | Closure costs, net 3 | $ mm | (38.9) | 99 | | Total capex | $ mm | 1,016 | 1,406 | | Economics (after-tax) | | | | | Total free cash flow, LOM 2 | $ mm | 901 | 1,577 | | Net Present Value (NPV5%) | $ mm | 502 | 943 | | Internal Rate of Return (IRR) | % | 20.7% | 22.1% | | Payback, from commercial production | yrs | 5.9 | 2.8 | | Average free cash flow, first 5 yrs 2 | $ mm | (63) | 296 | | Average free cash flow, LOM 2 | $ mm | 75 | 158 |

1. Total may not add up due to rounding. 2. Cash costs, all-in sustaining costs per ounce and free cash flow are non-IFRS measures or ratios. Refer to "Non-IFRS Financial Measures" at the end of this news release for more information. Total cash costs are presented on a per ounce payable basis inclusive of total operating costs mining costs, processing costs, site G&A costs, royalties, smelting, refining, and transports costs. AISC are presented on a per ounce payable basis and include cash costs plus sustaining and closure costs. 3. Closure costs are shown net of salvage value. 4. Pre-final investment decision capital costs total $38.6 million.

Figure 1: Payable Gold Production and AISC 1 Profile by Year

1. All-in sustaining costs per ounce is a non-IFRS ratio. Refer to "Non-IFRS Financial Measures" at the end of this news release for more information.

Figure 2: Free Cash Flow 1 by Year

1. Free cash flow is a non-IFRS measure. Refer to "Non-IFRS Financial Measures" at the end of this news release for more information.

ECONOMIC SENSITIVITY ANALYSIS

The Company used a base case gold price assumption of US$2,400/oz and a USDCAD exchange rate of 1.35 in its analysis and incorporated only Probable Mineral Reserves (as defined herein). Based on these assumptions, the Project generates an after-tax NPV5% of $943 million, an after-tax unlevered IRR of 22.1% and a payback of 2.8 years (from commercial production). The 2025 FS economics are most sensitive to fluctuations in the following inputs (in order of magnitude of impact): gold price, foreign exchange, operating costs, and capital costs.

| Table 3: Cariboo Gold 2025 FS – Economic Sensitivity Analysis, after-tax (base case in bold) | | | | | | Gold Price | NPV5% | IRR | Payback | Avg. FCF | | (US$/oz) | ($ mm) | (%) | (yrs) | ($ mm per yr) | | $1,800 | 251 | 9.9% | 5.3 | 65 | | $2,100 | 610 | 16.5% | 3.5 | 113 | | $ 2,400 | 943 | 22.1 % | 2.8 | 158 | | $2,700 | 1,275 | 27.2% | 2.2 | 204 | | $3,000 | 1,604 | 31.9% | 1.9 | 250 | | $3,300 | 1,934 | 36.3% | 1.7 | 296 | | $3,600 | 2,263 | 40.5% | 1.5 | 342 | | $3,900 | 2,592 | 44.5% | 1.3 | 388 | | $4,200 | 2,921 | 48.3% | 1.2 | 433 | | $4,500 | 3,249 | 51.9% | 1.1 | 479 |

| Table 4: Cariboo Gold 2025 FS – NPV5% Sensitivity Analysis, after-tax, $ mm (base case in bold) | | | | | | | Gold Price | USDCAD Exchange Rate | | | | | | (US$/oz) | 1.25 | 1.30 | 1.35 | 1.40 | 1.45 | | $1,800 | 59 | 166 | 251 | 335 | 419 | | $2,100 | 439 | 525 | 610 | 695 | 780 | | $ 2,400 | 750 | 847 | 943 | 1,040 | 1,136 | | $2,700 | 1,058 | 1,167 | 1,275 | 1,382 | 1,490 | | $3,000 | 1,364 | 1,484 | 1,604 | 1,724 | 1,845 | | $3,300 | 1,669 | 1,801 | 1,934 | 2,066 | 2,198 | | $3,600 | 1,974 | 2,118 | 2,263 | 2,408 | 2,552 | | $3,900 | 2,279 | 2,436 | 2,592 | 2,749 | 2,905 | | $4,200 | 2,584 | 2,753 | 2,921 | 3,089 | 3,258 | | $4,500 | 2,888 | 3,069 | 3,249 | 3,430 | 3,610 |

| Table 5: Cariboo Gold 2025 FS – IRR Sensitivity Analysis, after-tax, % (base case in bold) | | | | | | | Gold Price | USDCAD Exchange Rate | | | | | | (US$/oz) | 1.25 | 1.30 | 1.35 | 1.40 | 1.45 | | $1,800 | 6.2% | 8.3% | 9.9% | 11.5% | 13.0% | | $2,100 | 13.4% | 15.0% | 16.5% | 18.0% | 19.4% | | $ 2,400 | 18.9% | 20.6% | 22.1 % | 23.7% | 25.2% | | $2,700 | 23.9% | 25.6% | 27.2% | 28.8% | 30.3% | | $3,000 | 28.5% | 30.2% | 31.9% | 33.5% | 35.2% | | $3,300 | 32.8% | 34.6% | 36.3% | 38.0% | 39.7% | | $3,600 | 36.8% | 38.7% | 40.5% | 42.3% | 44.1% | | $3,900 | 40.7% | 42.7% | 44.5% | 46.3% | 48.1% | | $4,200 | 44.4% | 46.4% | 48.3% | 50.1% | 52.0% | | $4,500 | 47.9% | 49.9% | 51.9% | 53.8% | 55.7% | | | | | | | |

MINERAL RESOURCES AND MINERAL RESERVES

Vein corridors that make up the Cariboo Gold Project Mineral Resources and Mineral Reserves comprise a high-density network of mineralized quartz veins hosted within unmineralized sandstone. Individual mineralized veins within these corridors have widths varying from centimeters to several meters and strike lengths from a few meters to over 50 m. These corridors have been defined from surface to a vertical depth of 650 m, averaging 300m, and remain open for expansion at depth and along strike. Gold grades are intimately associated with quartz vein-hosted pyrite as well as pyritic, intensely silicified wall rock haloes in close proximity to the veins.

Figure 3: Vein Corridor in Lowhee Bulk Sample Area

Mineral Resources Estimate

The Mineral Resources estimate included in the 2025 FS have an effective date of April 22, 2025, and are reported exclusive of Mineral Reserves. Mineral Resources that are not Mineral Reserves do not have demonstrated economic viability. The Mineral Resources estimate have been updated to account for depletion in the Lowhee Zone, due to ongoing development and bulk sample activities, as well as for changes in costs and cut-off grade assumptions. This resulted in an aggregate increase of 3% of the total contained gold ounces in the Indicated Resources category and an increase of 8% of the total contained gold ounces in the Inferred Resources category as compared to the 2023 FS.

There is no certainty that Mineral Resources will be converted into Mineral Reserves. Mineral Resources include Inferred Mineral Resources which have had insufficient work to classify them as Indicated Mineral Resources. It is uncertain but reasonably expected that Inferred Mineral Resources could be upgraded to Indicated Mineral Resources with continued exploration.

| Table 6: Cariboo Gold 2025 FS – Mineral Resources Statement (April 22, 2025) | | | | | Category / Zone | Tonnage | Gold Grade | Contained Gold | | | (000's tonnes) | (g/t) | (000's oz) | | Measured | | | | | Bonanza Ledge | 47 | 5.06 | 8 | | Indicated | | | | | Bonanza Ledge | 32 | 4.02 | 4 | | BC Vein | 1,057 | 3.00 | 102 | | KL | 527 | 2.80 | 47 | | Lowhee | 1,333 | 2.76 | 118 | | Mosquito | 1,553 | 2.96 | 148 | | Shaft | 6,121 | 2.92 | 575 | | Valley | 2,718 | 2.70 | 236 | | Cow | 3,991 | 2.91 | 374 | | Total Indicated Resources | 17,332 | 2.88 | 1,604 | | Total Measured & Indicated | 17,380 | 2.88 | 1,612 | | Inferred | | | | | BC Vein | 596 | 3.17 | 61 | | KL | 2,514 | 2.53 | 205 | | Lowhee | 486 | 3.01 | 47 | | Mosquito | 1,883 | 3.08 | 186 | | Shaft | 7,457 | 3.44 | 826 | | Valley | 2,470 | 3.01 | 239 | | Cow | 3,368 | 2.78 | 301 | | Total Inferred Resources | 18,774 | 3.09 | 1,864 |

1. The independent and qualified persons for the Mineral Resources estimates, as defined by NI 43-101, are Carl Pelletier, P.Geo., and Tessa Scott, P.Geo. (Norda Stelo). The effective date of the 2025 Feasibility Study Mineral Resource Estimate is April 22, 2025. 2. These Mineral Resources, exclusive of the reserves, are not Mineral Reserves and do not have demonstrated economic viability. 3. The Mineral Resources estimate follows the 2014 CIM Definition Standards on Mineral Resources and Reserves and the 2019 CIM Estimation of Mineral Resources and Mineral Reserves Best Practice Guidelines. 4. A total of 481 vein zones were modelled for the Cow Mountain (Cow and Valley), Island Mountain (Shaft and Mosquito), Barkerville Mountain (BC Vein, KL, and Lowhee) deposits and one gold zone for Bonanza Ledge. A minimum true thickness of 2.0 m was applied, using the gold grade of the adjacent material when assayed or a value of zero when not assayed. 5. The estimate is reported for a potential underground scenario at a cut-off grade of 1.8 g/t Au, except for Bonanza Ledge at a cut-off grade of 3.5 g/t Au. The cut-off grade for the Cow, Valley, Shaft, Mosquito, BC Vein, KL, and Lowhee deposits was calculated using a gold price of US$2,400/oz; a USDCAD exchange rate of 1.35; an underground mining cost of $66.3/t; a processing and transport cost of $30.80/t; a G&A plus Environmental cost of $22.40/t; and a sustaining CAPEX cost of $45.6/t. No changes have been applied for the Bonanza Ledge. The cut-off grade for the Bonanza Ledge deposit was calculated using a gold price of US$1,700/oz; a USDCAD exchange rate of 1.27; an underground mining cost of $79.13/t; a processing and transport cost of $65.00/t; and a G&A plus Environmental cost of $51.65/t. The cut-off grades may be re-evaluated in light of future prevailing market conditions (metal prices, exchange rate, mining cost, etc.). 6. Density values for Cow, Shaft, Lowhee, and BC Vein were estimated using the ID2 interpolation method, with a value applied for the non-estimated blocks of 2.80 g/cm 3 for Cow, 2.78 g/cm 3 for Shaft, 2.74 g/cm 3 for Lowhee, and 2.69 g/cm 3 for BC Vein. Median densities were applied for Valley (2.81 g/cm 3 ), Mosquito (2.79 g/cm 3 ), and KL (2.81 g/cm 3 ). A density of 3.20 g/cm 3 was applied for Bonanza Ledge. 7. A four-step capping procedure was applied to composited data for Cow (3.0 m), Valley (1.5 m), Shaft (2.0 m), Mosquito (2.5 m), BC Vein (2.0 m), KL (1.75 m), and Lowhee (1.5 m). Restricted search ellipsoids ranged from 7 to 50 g/t Au at four different distances ranging from 25 m to 250 m for each deposit. High grades at Bonanza Ledge were capped at 70 g/t Au on 2.0 m composited data. 8. The gold Mineral Resources for the Cow, Valley, Shaft, Mosquito, BC Vein, KL, and Lowhee vein zones were estimated using Datamine StudioTM RM 1.9 software using hard boundaries on composited assays. The dilution halo gold mineralization was estimated using Datamine StudioTM RM Pro 1.11. The OK method was used to interpolate a sub-blocked model (parent block size = 5 m x 5 m x 5 m). Mineral Resources for Bonanza Ledge were estimated using GEOVIA GEMSTM 6.7 software using hard boundaries on composited assays. The OK method was used to interpolate a block model (block size = 2 m x 2 m x 5 m). 9. Results are presented in situ. Ounce (troy) = metric tons x grade / 31.10348. Calculations used metric units (metres, tonnes, g/t). The number of tonnes were rounded to the nearest thousand. Any discrepancies in the totals are due to rounding effects. Rounding followed the recommendations as per NI 43-101. 10. The qualified persons responsible for this section of the technical report are not aware of any environmental, permitting, legal, title, taxation, socio-economic, marketing, political, or other relevant factors that could materially affect the Mineral Resource estimate other than those disclosed in this news release and in the Technical Report.