My thinking would be nip/bit by a fish did this. I really would not suspect wood.
And of the fish mentioned, neon tetras may be responsible. Corydoras pygmaeus do not have teeth so that lets them out. Nannostomus eques may get annoyed at fish up in their territory, but this species is relatively mild-mannered, and their very small mouths which are permanently open would be unlikely, but there is no certainty with any fish. Paracheirodon innesi does have teeth, lots of them, and if one was especially aggressive for any number of reasons it could well inflict this damage on an oto.
I had to move my Nannostomus beckfordi and N. trifasciatus because they nipped any fish that came up to the surface, including otos. But those two species are much more feisty, but still, nothing is certain when dealing with live creatures. In some two decades of keeping N. eques I have never seen attacks on other species, only themselves, but with no damage being done.
