The betta should get used to the oto in awhile. Basically all mine did was pick for a while, flare for a while then carry on as if normal. But the oto likes company and you may find that it will just do nothing (doesnt happen often) cause its lonely. So if the oto seems to just hang around then that may be the reason. If the betta thinks its lunch then just turn off the lights for a day or so it seemed to calm mine down and when the lights went back on the 3 of them got along famously. HTH