Betta Fish: Potential Tankmates?

What are some potential tankmates for this guy? Preferably I'd like to add a few more fish but I'm not going to push it if it doesn't look like it'll work.

There are/where 3 cherry shrimp but I haven't seen them more then twice since I added Oscar.

There are plenty of hiding spots in the rocks and the occasional algae wafer I drop always seems to disappear so I think they're still in there just hiding

Oscar was interested in them but never gave chase when they ran away from him but he's very energetic on average.

Tank is 20 gallons and fully cycled at this point with the plants showing predominant growth