A 5 gallon is a great tank size, but sadly it is too small for actual fish tankmates. Both Ottos and Neons are schooling/shoaling fish who need to be in larger groups of 5-6+, which is just too much for a 5 gallon. There just isn't enough swimming space.
Ottos can be a little finicky anyway. It can be hard to get them to eat and they're really sensitive to water quality. I believe this is because most of them are wild-caught...
With a single betta in a 5 gallon, you're pretty limited with tank mates. You could do a couple of snails or a few shrimps(Ghost shrimp are the ones most commonly sold, though you can look for Red Cherry shrimp or a few other species online if you want something a tad more colorful.....careful though! Bettas often make shrimps snacks). Or if you'd be willing to deal with the tricky feeding, you could go with a pair of ADFs(African Dwarf Frogs).
Another option is dividing the tank in half for a second betta.....but thats about it I'm afraid.