If your fish are scratching all the time, then you most propably have a
parasite. If they are scratching occasionally (once in a while) , then they
just have an itch and nothing to worry about.

If your fish are scratching regulary and your Otos "attack" them, I would
certainly get my magnifying glass out. Sounds like you do have a parasite
and the Otos might just be after them.

We have several different versions of Otos and Paraotos, none ever attack
the fish or suck on them. Otos tend to do better in groups, my Paraoto
killed of all his mates, so he is now in a little tank where there is only
space for one Oto anyway. Paraotos seem to be more active during the night
as well (I used to think I lost him several times, as I couldn't find him in
a 5G tank for up to 3 weeks, but then suddenly he would re-appear and hang
on the front glass...).

You can find "your" species he

http://www.planetcatfish.com/core/index.htm

(click on cat e-log)

Greetings
Sabine