OBS's audio is not that great to start with. The quality is there, but the workflow is very much a pile of ad-hoc band-aids on top of an originally good design for a very limited purpose, which is not what you're doing with it.
There is exactly one audio output from OBS, besides the direct stream to YouTube, Facebook, Twitch, or other service, and the recording file. That one audio output is the Monitor. The Monitor is intended to be used for headphones, but because there's nothing else, it tends to get abused for a lot more than that. It's still a single output, so you can't have things come out of there and then be separate.
So, to send audio from OBS to Zoom or anything else that takes a mic input, you'll need to send it out the Monitor from OBS, and then send the Monitor to a loopback device of some kind.
- That loopback could be something you install, which creates a virtual speaker that you connect the Monitor to, and a virtual mic that you connect the other app to.
- Or it could be a physical device, that you connect the other app to the output of.
The latter is technically still a loopback, but it's provided directly by the operating system for every audio output device, mostly for that exact purpose.