Opposing Force was designed for the GoldSrc engine. To make it appear correctly in the Xash3D menu (instead of just showing "Half-Life"), you need to verify the game library file.
The intersection of and the Xash3D engine represents a fascinating chapter in community-driven game preservation and cross-platform porting. While Valve's original masterpiece was designed for Windows, the Xash3D project—specifically the Xash3D FWGS (Flying With Gauntlet Software) branch—allows this classic expansion to live on modern hardware like Android, macOS, and Linux. The Technical Legacy of Opposing Force halflife opposing force xash3d download full
Xash3D fixes many of the "buffer overflow" and resolution scaling issues found when trying to run the original 1999 retail version on Windows 10 or 11. Troubleshooting Common Issues Opposing Force was designed for the GoldSrc engine