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I am certainly no expert so I have no idea, but is it possible it is pregnant? I have four of them and come to think of it, I have no clue as to what their breeding habits are.
I'm no expert but my experience with this is that I brought 15 as a clean up crew because the tank was getting too dirty with algea growth sadly one by one they got real fat like the one in your pic and eventually died not sure what it was or why but it did look exactly the same as this guy
Looks to me to be just fat. I have one seriously fat oto, fatter than yours actually, and has been for a very long time. I recently bought a few more and a couple have got bellies like yours and are completely fine. unless you were seeing your fish swimming oddly or something else I would just take your fat oto as what it is, a fat oto
Pictures from a couple months ago of my fatty, he's a little bigger now