22:02:18.761: coreaudio: device 'USB Audio Device' initialized
22:02:18.762: coreaudio: failed to find device uid: AppleUSBAudioEngine:Loud Technologies Inc.:Onyx Blackjack:14100000:2,1, waiting for connection
OBS isn't apple to access this audio device-- possibly a permissions issue. You may wish to try giving it permissions in the Security and Privacy system prefs panel, or launch the app from the command line. In Mojave, Apple added a security setting for access to audio/video devices.
It's also possible that the driver is just not up to date; the support page for the device indicates it was compatible with MacOS X 10.4-10.6. Plenty of older standard devices absolutely should just work, though, so I'd lean towards it being a permission problem.