Angels often get a bad wrap about eating little fish. This just isn't totally correct. Now I will say if by chance someone were to get a wild caught angel or some of the wild caught mixes they could have some real issues with keeping small fish. But most angels available today don't get near as large as the original wild caughts. All that being said the only issues I've found from angels in the 30+ years I've kept them has almost exclusively come from very aggressive large mature adult males. They just sometimes get so aggressive they go after everything and little fish are easy targets. BUT this is not very common. Right now I have 12 adult breeding angels in a tank with alot of small fish including 18 oto's and about 20ish very small cardinal tetra's. There are also a few Rachovii killifish in the tank that are very small also. The angels don't see other fish as food but shrimp are fair game. Alot depends on the individual personalities of the angels but for the most part when angels are raised around little fish they tend not to pay any attention to them. So IMO
getting Oto's would be fine. One thing tho, after acclimating them, when your ready to put them in the tank, feed the angels at the opposite end of the tank a second before adding the oto's. Angels are curious and it's best to distract them when adding new fish of any kind.