This is the game installation for the Half-Life: Opposing Force SDK.
Place this in your Half-Life directory under the directory name half-life-op4 and restart Steam to see the SDK in your games list.
You will need to have Half-Life: Opposing Force installed in order to play this.
You do not have to copy any files from Opposing Force to play this.
You will need to have the Visual Studio 2017 x86 redistributable installed in order to play this. You can download it here: https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads
Download vc_redist.x86.exe and install it.