You can try a variety of different veg and see what they seem to prefer as some like one thing more than another.
Typical favourites are: slices of courgette or cucumber, spinach, Romaine lettuce, broccoli.
Otos will often ignore any veg for the first 12 hours or so until it begins to soften so to help with that, blanch the veg by popping into the microwave for 30 seconds on a saucer with a little water.
Use a seaweed clip to hold lettuce or spinach, and for other veg either push a piece of plant weight through to make them sink to the bottom, or fix the veg to a stone with a rubber band. For just a few Otos the screwcumber is a bit big and you'd waste a lot of veg as you'd need to use a lot more than they could eat with one of those attached.
You can leave the veg in for up to 48 hours, but remove sooner if it starts to go mushy or break apart as it will foul the water.
Otos really need fesh veg most days if there isn't a lot of algae in the tank. They can have occasional algae wafers too, but not too often as they usually have protein added and Otos aren't designed to cope with it. Too many algae wafers can cause them to become bloated and unwell.