Osisko Mining Releases Discovery 1 Preliminary Results

TORONTO, Sept. 11, 2019 - Osisko Mining Inc. (OSK:TSX. "Osisko" or the "Corporation") is pleased to provide an update on the Discovery 1 drill hole. Discovery 1’s principal exploration target is the interpreted outer shell and centre of a possible porphyry intrusion feeding the Windfall-Lynx gold system, at depths of approximately 3,000 metres to 3,500 metres from surface. The drill hole was designed to also target potential depth extensions of two known mineralized zones (the Underdog Zone and Triple 8 Zone). Preliminary results from Discovery 1 include intersections from the targeted Underdog and Triple 8 extensions, noted below. Approximately 2,300 metres down-hole length has been completed, with approximately 1,200 metres left to reach the principal exploration targets.

The initial attempted hole (OSK-W-19-1944) was started on May 10th and was ended at 812 metres length due to excessive deviation. A second attempt (OSK-W-19-1970) was commenced on May 30^th, 2019 and has so far completed approximately 2,300 metres of the planned 3,500 metres. Eleven dip corrections have been done at regular spacings up to 2,050 metres using retractable wedges to control deviation.

A new high-grade extension of Underdog Zone was intersected at approximately 1771 metres down-hole depth (14.1 g/t Au over 2.1 metres), 500 metres from the nearest Underdog resource wireframe. This new intercept is a significant new down-plunge extension of the zone, which will be followed up with additional drilling later in the year.

Discovery 1 has also intersected significant new mineralization in the Triple 8 corridor (9.58 g/t Au over 7.6 metres) at 2,191 metres down-hole, adding information and extending the known mineralization in this zone by approximately 100 metres.

Osisko President and Chief Executive Officer John Burzynski commented: “Discovery 1 is already paying dividends with a new 500 metre high-grade extension of the Underdog Zone and a new wide intercept in Triple 8. We are now moving to the really interesting part of this hole and are hoping to get a good look at the potential roots and feeder system of the Windfall deposit as Discovery 1 progresses towards the main deep targets.”

Maps showing hole locations and full analytical results are available at www.osiskomining.com.

Underdog Extension
The first interval intersected 5.75 g/t Au over 3.2 metres. Mineralization includes local visible gold, 7% pyrite stringers, clusters and locally disseminated hosted in a moderately chloritized, moderately bleached and locally moderately to strongly silicified andesite in contact with felsic porphyritic intrusions with trace large quartz eyes. The second interval returned 62.3 g/t Au over 1.0 metre, with mineralization consisting of centimeter-scale quartz veins in a moderate chlorite and weakly silica and sericite altered andesite. The third interval returned 14.1 g/t Au over 2.1 metres lower down in the Underdog corridor. Mineralization consists of 3% pyrite stringers and 1% disseminated pyrite associated with pervasive silica flooding and a quartz-carbonate vein hosted in a moderately sericitized porphyritic felsic intrusion.

The intervals are located approximately 500 metres down-plunge of the defined Underdog resource and the first two intervals occur in a broader zone of low-grade mineralization and alteration located between 1614.0m – 1686.0m.

Triple 8 Zone

OSK-W-19-1970 intersected 9.58 g/t Au over 7.6 metres in the Triple-8 corridor. Mineralization includes local visible gold in clusters and disseminated associated with patchy silica alteration, up 20% disseminated and clustered pyrite with trace chalcopyrite stringers and up to 1% pyrrhotite hosted in a moderate sericite and silica altered andesite with weak foliation. This intercept is located 100 metres from a prior intersection in OSK-W-18-1783 (16.0 g/t Au over 2.3 metres, see Osisko news release dated February 5, 2019).

Qualified Person
The scientific and technical content of this news release has been reviewed, prepared and approved by Mr. Louis Grenier, M.Sc.A., P.Geo. (OGQ 800), Project Manager of Osisko's Windfall Lake gold project, who is a "qualified person" as defined by National Instrument 43-101 – Standards of Disclosure for Mineral Projects ("NI 43-101").

Quality Control and Reporting Protocols
True width determination is estimated at 55-80% of the reported core length interval for the zone. Assays are uncut except where indicated. Intercepts occur within geological confines of major zones but have not been correlated to individual vein domains at this time. Reported intervals include minimum weighted averages of 3.0 g/t Au diluted over core lengths of at least 2.0 metres. All NQ core assays reported were obtained by either 1-kilogram screen fire assay or standard 50-gram fire-assaying-AA finish or gravimetric finish at (i) ALS Laboratories in Val d'Or, Québec, Thunder Bay, Ontario, Sudbury, Ontario or Vancouver, British Colombia, or (ii) Bureau Veritas in Timmins, Ontario. The 1-kilogram screen assay method is selected by the geologist when samples contain coarse gold or present a higher percentage of pyrite than surrounding intervals. Selected samples are also analyzed for multi-elements, including silver, using an Aqua Regia-ICP-AES method at ALS Laboratories. Drill program design, Quality Assurance/Quality Control ("QA/QC") and interpretation of results is performed by qualified persons employing a QA/QC program consistent with NI 43-101 and industry best practices. Standards and blanks are included with every 20 samples for QA/QC purposes by the Corporation as well as the lab. Approximately 5% of sample pulps are sent to secondary laboratories for check assay.

About the Windfall Lake Gold Deposit
The Windfall Lake gold deposit is located between Val-d'Or and Chibougamau in the Abitibi region of Québec, Canada. The mineral resource defined by Osisko, as disclosed in the Windfall Lake Technical Report (as defined below) and November 27, 2018 Lynx resource update, comprises 2,874,000 tonnes at 8.17 g/t Au (754,000 ounces) in the indicated mineral resource category and 10,352,000 tonnes at 7.11 g/t Au (2,366,000 ounces) in the inferred mineral resource category. For details regarding the key assumptions, parameters and methods used to estimate the mineral resources presented in respect of the Windfall Lake gold project, please see the technical report entitled "Technical Report and Mineral Resource Estimate for the Windfall Lake Project, Windfall Lake and Urban-Barry Properties" and dated June 12, 2018 (effective date of May 14, 2018), which has been prepared by InnovExplo Inc. from Val-d'Or, Québec (the "Windfall Lake Technical Report") and the press release “Osisko Releases Mineral Resource Update for Lynx” dated November 27, 2018, which has been prepared by Osisko and reviewed and approved by Micon International, Ltd. from Toronto, Ontario. The Windfall Lake Technical Report and press release are available on Osisko's website at www.osiskomining.com and on SEDAR under Osisko's issuer profile at www.sedar.com. The Windfall Lake gold deposit is currently one of the highest-grade resource-stage gold projects in Canada. Mineralization occurs in four principal zones: Lynx, Zone 27, Caribou and Underdog. All zones comprise sub-vertical lenses following intrusive porphyry contacts plunging to the northeast. The deposit is well defined from surface to a depth of 900 metres and remains open along strike and at depth. Mineralization has been identified 30 metres from surface in some areas and as deep as 2,000 metres in others, with significant potential to extend mineralization down-plunge and at depth.

About Osisko Mining Inc.
Osisko is a mineral exploration company focused on the acquisition, exploration, and development of precious metal resource properties in Canada. Osisko holds a 100% interest in the high-grade Windfall Lake gold deposit located between Val-d'Or and Chibougamau in Québec and holds a 100% undivided interest in a large area of claims in the surrounding the Urban Barry area and nearby Quévillon area (over 2,700 square kilometres).

Cautionary Note Regarding Forward-Looking Information
This news release contains "forward-looking information" within the meaning of the applicable Canadian securities legislation that is based on expectations, estimates, projections and interpretations as at the date of this news release. The information in this news release about the Windfall Lake gold deposit being one of the highest grade resource-stage gold projects in Canada; the significance of results from the new infill drilling and ongoing drill definition and expansion program at the Windfall Lake gold project; the significance of assay results presented in this news release; the deposit remaining open along strike and at depth; potential depth extensions of the mineralized zones down-plunge and at depth; the actual mineralization of local visible gold; the current 800,000 metre drill program; the type of drilling included in the drill program; potential mineralization; the potential to extend mineralization up and down-plunge and at depth at the Windfall Lake gold deposit; the ability to realize upon any mineralization in a manner that is economic; the ability to complete any proposed exploration activities and the results of such activities, including the continuity or extension of any mineralization; and any other information herein that is not a historical fact may be "forward-looking information". Any statement that involves discussions with respect to predictions, expectations, interpretations, beliefs, plans, projections, objectives, assumptions, future events or performance (often but not always using phrases such as "expects", or "does not expect", "is expected", "interpreted", "management's view", "anticipates" or "does not anticipate", "plans", "budget", "scheduled", "forecasts", "estimates", "believes" or "intends" or variations of such words and phrases or stating that certain actions, events or results "may" or "could", "would", "might" or "will" be taken to occur or be achieved) are not statements of historical fact and may be forward-looking information and are intended to identify forward- looking information. This forward-looking information is based on reasonable assumptions and estimates of management of the Corporation at the time such assumptions and estimates were made, and involves known and unknown risks, uncertainties and other factors which may cause the actual results, performance or achievements of Osisko to be materially different from any future results, performance or achievements expressed or implied by such forward-looking information. Such factors include, among others, risks relating to the ability of exploration activities (including drill results) to accurately predict mineralization; errors in management's geological modelling; the ability of Osisko to complete further exploration activities, including drilling; property interests in the Windfall Lake gold project; the ability of the Corporation to obtain required approvals and complete transactions on terms announced; the results of exploration activities; risks relating to mining activities; the global economic climate; metal prices; dilution; environmental risks; and community and non-governmental actions. Although the forward-looking information contained in this news release is based upon what management believes, or believed at the time, to be reasonable assumptions. Osisko cannot assure shareholders and prospective purchasers of securities of the Corporation that actual results will be consistent with such forward-looking information, as there may be other factors that cause results not to be as anticipated, estimated or intended, and neither Osisko nor any other person assumes responsibility for the accuracy and completeness of any such forward-looking information, Osisko does not undertake, and assumes no obligation, to update or revise any such forward-looking statements or forward-looking information contained herein to reflect new events or circumstances, except as may be required by law.

For further information please contact:
John Burzynski
President and Chief Executive Officer
Telephone: (416) 363-8653