Can you get a pic of the oto's belly by any chance?
A few questions: what is the fish's diet? Do you feed veggies , algae wafers or any other foods? Is the fish acting normally? Is there any redness on the belly or near the anus?
It could be gravid, or it could just be bloated. You could try feeding some shelled green peas for now - if it is an intestinal problem then the peas will help to get things moving. If it is gravid, there's a small chance that you'll get fry. How many otos are in the tank?
Otocinclus fry are very difficult to raise, but on occasion fry do appear in people's tanks and survive. If you do end up with fry I can give you some ideas that may help.