Most Oto's are wild caught & often not in great condition & starving when they reach the Lfs.
When you go to buy them take time to watch them, look for a nice rounded tummy not a flat or sunken one.
The more Oto's you have the happier they are & you'll see more natural behaviour.
They spend a lot of time grazing on wood, plants & the glass for algae, but do not expect them to keep your glass sparkling, they won't.
Oto's are sensitive to water quality, they can also be slow to accept commercial food.
When trying to get them to take wafers or veggies always put them in the same place & they'll eventually get the message it's food, they are nocturnal so any food you put in should be left in the tank overnight, they don't Hoover up food like other fish & take their own sweet time about it.
You can grow algae for them on rocks or wood by leaving them in a tub of tank water on a sunny window ledge