Howdy! Unfortunately on MacOS you cannot record desktop audio directly from OBS (i.e you need a separate 3rd party application to run it). For me, I use an app called "Background Music for Mac." This program is not intended for this purpose, but I found that it actually works really well.
What I found was that this handy little app had a built in virtual input (meaning you can put it into OBS, zoom, discord, etc). The background music app allowed for me to manually adjust the volume of every single program on my computer (hence the name "background music." This feature is already natively on windows, but unfortunately not on Mac). Since I can adjust the volume of every single program and device, that also meant my headset and speakers. From there, I added my headset output as the programs main input (meaning any noise that played through my headset would be directed into OBS). Then also using background music, I could mute specific programs so that they wouldn't go onto the stream (I did this with things like iMessage, FaceTime, discord, etc so that they would not be picked up in OBS). It is a little complicated, but I found that this accidental feature works better than any other program I've tried.
Hopefully this helps!
