Catalina on a 2012 iMac means you're stuck with OBS 27.2.4, that's the newest version that still supports 10.15. You can grab it from github.com/obsproject/obs-studio/releases/tag/27.2.4 -- the current OBS 32 requires Ventura or later. With 8GB and that i5 you should be fine for basic streaming, just keep output at 720p and use a faster x264 preset so the CPU doesn't choke.
