Half-Life: Opposing Force is now available as a Beta. This beta adds Linux and OS X support.
This beta involves significant changes for all platforms and your testing is appreciated, please report any bugs to our GitHub page, https://github.com/ValveSoftware/halflife/issues.
Linux and OS X users can simply install the Half-Life: Opposing Force application to access the beta. For Windows users right click the game in your Library, choose properties and then go to the Beta tab. Select the SteamPipe beta to start testing. Under windows to opt out of testing simply deselect the beta option on this same page. If you opt into the Beta under Windows please use the Local Files tab in Steam to validate your install once you have opted into the beta.