Otos are very safe tank mates, they don't do much to aggravate the betta.
Cories are GREAT fish, and a few dwarfs would be happy in a ten gallon. I would highly suggest a bottom dweller, as they would (most likely) stay more out of his way, and thus he would be more likely to accept them.
Keep in mind though, that whatever you get may not work out. Some bettas just don't want tank mates.
One thing you can do in acclimatizing:
1) quarantine your new fish for at least 2 weeks, if not 4.
2) put your betta in a temporary home (even if its just the cup he came in, floating in another tank for temp control)
3) rearrange his decor a bit
4) move your betta's new roomies into the tank
5) replace the betta in his tank.
Thats the method that is most likely to garnish success over the whole invaders-to-his-territory thing. If he's last in, he might figure its the new fish's tank, so he'd better play nice.
Warning: Although I read a lot, I'm new to this, thus take my suggestions with a grain of salt! ;)