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.
Opposing Force is an expansion. It requires base Half-Life assets (textures, sprites, core sounds) to run. Xash3D needs the valve folder from the original game. halflife opposing force xash3d download full
There are two ways to do this: the Portable Build (simplest for Windows) or the Android Version. Select your platform below. Opposing Force was designed for the GoldSrc engine
Before we dive into the "download full" aspect, you need to understand the tool. Opposing Force is an expansion
Xash3D is a custom, open-source engine designed to be a drop-in replacement for Valve’s original GoldSrc engine (the engine that runs Half-Life, CS 1.6, and Opposing Force). It is not a mod; it is a re-implementation of the engine itself.
Crucial Legal Note: Xash3D is an engine that contains zero copyrighted game data. You cannot download "Half-Life: Opposing Force" for free via Xash3D alone. You must own the game files. This guide assumes you own a legitimate copy (usually via Steam).