At a glance
Half-Life: Opposing Force has a Mac version: Steam sells it for macOS. The Mac requirements list only Intel: on Apple Silicon Rosetta 2 runs it, and old Mac versions sometimes do not start on current macOS. Try it in Steam for Mac first.
Half-Life: Opposing Force came out on Steam on 1 November 1999, made by Gearbox Software.
What your Mac needs
The Mac requirements list only Intel.
Minimum macOS in the Steam requirements: 10.6.3.
Steam requirements list at least 1 GB of memory for Half-Life: Opposing Force.
Allow about 4 GB of disk space for the Mac version.
How to play
Open Steam for Mac and install Half-Life: Opposing Force as usual.
If the Mac version will not start, you can install the Windows version through Uncork: download Uncork, sign in to Steam and install Half-Life: Opposing Force from there. We have not tested it ourselves.
The Windows version through Uncork needs a Mac with Apple Silicon (M1 or newer).