First, if you're not using any custom code in your mod, you don't even need a dll or client.dll in your mod folder. Just reference the main HL dll by using: gamedll "..\valve\dlls\hl.dll"
Second, make sure Half-Life is up to date, and then try using that dll and client.dll.
This bug popped up a while back when they first moved HL to Steampipe. I believe they fixed it at some point, at least with hl.dll.
Last time I tried setting up an Opposing Force "mod" (that is, copying all the files from the gearbox folder to a mod folder and running it), this bug was still there. It seems that at least for Op4, there is something hardcoded that expects the gearbox directory, and if you use the Op4 dll in your own mod folder, you get the limited view rotation like you're getting now.