Settings -> Audio -> Monitoring Device (or something like that, from memory), immediately below the global sources.
This is the only physical audio output from OBS. It would be wonderful if it had several, like another copy of the same selection for each track (total of 7 now), but at the moment, that's it.
Also be aware that OBS has trouble driving some sound cards, while others are okay. If yours does have a problem, it'll manifest as an ever-increasing delay until it's unusable. Disrupting the stream to that card resets the delay, but it continues to drift out of sync again.
You can choose a different card and change it back again, or you can unroute a problematic source from the monitor and route it back again, or you can mute and unmute the source. If you use the mute option, it'll affect the stream too, which probably had nothing wrong with it, unless you have two copies of the same source: one to stream and leave alone, and one to monitor and "blink" every so often to get it back in sync.
Virtual sound cards don't have this problem (but might have another one, based on my recent experience), so you can send the monitor to one of those and then "bounce" it from there to the real destination, but then you need a virtual thing that can do that.