OPULoaderShim injects the op2ext mod loader into Outpost 2. It works by shimming the winmm.dll system DLL (with a fallback mode replacing helphook.dll) which Outpost2.exe requires, and then passively loads op2ext.dll.
The use of OPULoaderShim does not result in any functional differences with winmm.dll (or helphook.dll) interfaces.
Note that while OPULoaderShim and therefore op2ext are unconditionally loaded into Outpost 2, op2ext does not activate unless the /OPU command switch is passed to the game when loaded. This allows the unmodified game to still be played.