For the sake of convenience I've included obs.lib in this repository. (It'd be really nice if it was included with official OBS builds since it's only 300 KB, but w/e.)
It was built by following the official build instructions using CI/build-windows.ps1. (At glance this does not seem to pollute the environment or anything untoward.)
After building, copy obs-studio/build64/libobs/RelWithDebInfo/obs.lib to this directory.