I think I may know whats going on here:
It is normally suggested you have an Oto -per- 10 gallons. So a 55 gallon tank that you say is, overrun with algae? And two little guys to do the job? They may be happy little buggers as they are. Hold your horses, and see if they start to pick away at it. The other way you know your Otos are happy is, if they look like they have full bellies and are pooping like crazy.
Now would you happen to have any plants? It is often noted in Oto profiles that they eat the algae off of plants, is that your problem or is it on the glass? Because if it's on the glass then feel free to scrape it off. That's one of the #1 ways to combat algae.
My best advice is wait and see what happens. If it's really that bad, scrape the glass but leave algae on the plants, decorations, or whatever you have in there. They'll feed off of that. I've never found a Oto to be fond of a algae wafer...