I have found that the dwarf puffers won't usually bother any bottom dwelling fish. Mine didn't bother my pleco or my cories, so I would assume your oto is fine. While I'm on the subject, does anyone know how well the zebra oto's fare in the aquarium? They are reletively new to the aquarium trade but I am really interested in getting a couple (but for $9 a piece I want to make sure that they will make it!).