I think what might be causing the confusion is that with the way the models are sometimes packaged, you tend to get the impression that there is one location you install to which will affect all your HL games and mods. Like Minotaur said above me, most installation packs like this will have folders. The best thing to do is:
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In the case of your Half Life, look in the “half-life” folder for a folder called “valve” in there should be folders for models, sounds, and that type of thing. If you drop the models folder from your installation pack into the “valve” folder, then Windows will usually ask you if you want to merge the folders, and from there it’s just a matter of agreeing to merge and overwrite the original files.
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In the case of opposing force, you’d want to look for the “gearbox” folder, instead of the valve folder. In this case you follow the same steps that you did with the valve folder for half-life. Theoretically, I would imagine that blue shift WOULD work the same way, but there’s some sort of weird block that Steam has that won’t let you install new models to Blue Shift. For this, I use the Blue Shift: Unlocked mod that recreates Blue Shift as a Half Life mod, and for that you’d use the steps below.
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Any goldsource mod you install (at least if you’re using steam) will have the main mod folder located in the half-life folder (along side the valve folder from the first option i mentioned). The models and sounds folders should be located directly in the main mod folder, so you should be able to just drop the models folder right into it, and get the same merging and overwriting options.
So for each game or mod that you want to change how the models look, you’ll want to install the models upgrade pack for each one separately.