You could try a bit of well cooked spinach leaf, though it may take a few tries before they eat any. I'm really fond of Otos.. it's very sad the way they are handled in the fish industry.They are caught using cyanide to stun them, then held and not fed for awhile.
This is a real tragedy because they rely on bacteria to digest the cellulose which is what makes up the cell walls in the algae they eat, and that allows them to get the nutrients inside the cells once they break down. Without a steady intake of food, the bacteria tend to die off, so by the time they get to a lfs
, they are starving.
Even if they eat 'til they are bulging, without the gut flora they need, they can't digest their food. This is why so many die soon after reaching stores and more die in the first two months in a tank at home. If they survive two months, you are usually home free, and they can live many years. They are not truly fragile, it's just that they need a steady food intake and once caught the whole system gets destroyed. Only the strongest typically survive.
If you can ever get them tank bred, there are no real problems with them but only a few are bred in tanks, the vast majority are wild caught as has been said. None are bred commercially as yet.
I grow algae on rocks for mine. I put rocks in a jar, add tank water, leave it in a sunny window or right out in the sun, and let green fuzz grow. I top up with tank water as needed. Does not go swampy, possibly because I do run an airstone in the jar. Not all rocks work for this. Some won't grow any green, but marble chips are really, really good for the purpose. Once they are green on the side facing the sun, I drop them in the tank. They will be almost snow white again in a day or so and I take them out and drop them back in the jar.
I have also taken hair algae I was struggling with at the time and pureed it. I am not sure the Otos ate any, but the shrimp sure did.
If they are not eating algae wafers now, they will eventually. All of mine ate them within a few days of arrival, if not when they got home. I will not buy an Oto that has not been in the store for a minimum of a week, preferably two or more. The mortality rate is too high with freshly arrived fish.