ah, found out that even though the app didn't seem to be running, it was running as a process in Activity Monitor, so I shut down my computer and rebooted.
I then started OBS again, and for a second I saw the OBS screen and the app seemed to be running, but it quit right away, and this time again, the process was still running in Activity Monitor.
I also tried completely removing OBS (using AppCleaner) and then reinstalling, but that leaves me with the same problem....