I think it depends on the individual Oto. I have one in my tank, and I supplement with algae wafers, but I have noticed him munching on the occasional fallen flake food and even, albeit rarely, bloodworms.
There is really no guarantee that an Oto will not eat eggs, but I think it's somewhat doubtful.
Also, Oto's are reputed to do better in groups, but I have found that my individual Otocinclus has done just fine on his own. He's doubled in size, and is quite active. He even recovered from a small fungal infection without any medicinal assistance.
Just be careful not to put too many in a 20 gallon. These guys require a lot more algae than most people realize, and I believe that may be why they are known to suddenly perish in otherwise healthy tanks, for no apparent reason.
If you go with a small group, I wouldn't get more than 2 or 3, and would still supplement their diet with algae wafers. My Oto also enjoys the occasional shelled pea.