Yes, a few otos might be good if you add some live plants too.
The problem is that a 5 gallon tank is just too small. If you had bought a 10 gallon, I would suggest a nice school of corys to go with your betta, but with a 5 gallon, even 3 corys would be pushing it with a betta.
The thing you want to prevent is the development of conditions that could lead to bacterial infections since this is the cause of much trouble for both kinds of fish. Overcrowding is one of these causes and even if it's cycled and well filtered, you would still need to do twice weekly water changes (with 3 corys) to be on the safe side.
If, however, you can commit to that amount of work, it would be worth a try. Corys and bettas are a great combination.