I have had the same 4 otos (in my 10g) for over a year now...here's some tips:
When you get the replacement oto, look for the largest, most energetic one at the pet store. Also make sure his belly is rounded. Don't pick a small oto with a slim belly....they have more chance of dying.
Acclimate him SLOWLY to your tank and leave the tank lights off for the first 24 hours. This gives them time to relax and settle....my otos are very sensitive to light.
Don't worry if you don't SEE them eating....go by the roundness of their bellies instead.
I've never known otos to eat flakes.
I don't ever clean my tank (I let the otos do it). This insures that they are getting adequate algae most of the time. When I notice the bellies getting slimmer, I'll cut a small piece of cucumber/zuchinni length wise, take off the seeds, microwave it for approx. 1 minute, let it cool and anchor it down in the tank using a veggie clip or decorations... My otos seem scared of the veggie if it's floating around the tank...they like it stationary. (Unlike my pleco, who "rides" the cucumber around my larger tank).
With 4 otos in a 10g, I tend to give them veggies once every 2 weeks or so, but your mileage may vary.
(My tank is near a window and tends to breed algae at a quick pace)
I would encourage you not to give up on otos....oh! another thing is if you can have a total of 3 or more otos, they will be more active. They are more comfortable and secure in groups.
Good luck and HTH