I've never had Otos go to the surface before. What are the tanks parameters & how did you introduce them to the tank? Did you just pour them from the bag or did you drip acclimate them over several hours?
I also have Oto's and have never seen them sit up top. I would try like Nutter said and do a water test.If you I know Oto's are kinda fragile when introducing them to a new tank and should be acclimated very slowly and not just dumped in. Hope you have good luck!