Hi, oto's should only clean the plants, I've never come across them eating them but to answer you - they will eat Courgette or cucumber although it can be difficult to get them to notice it at food. Perseverence is the key. Don't leave it too long, I don't leave it in my tank for longer than 48 hours - although by then my oto's, BN and SAE's have devoured it anyway.
There are two ways to get it to the oto's. You can either cut a small piece, slit it and pass a plant weight through so that it stays in one place on the floor of the tank (some people weight it down with an old spoon aswell) or if you a plastic clip you can clamp it in that, all some string through it and suspend it in the tank. The oto's should take it either way
Good luck
EDIT: I forgot to say, my oto's love New Era plec pellets. I've not had any success in getting them to eat any other kind or brand and I've tried a lot but the New Era one for some reason gets fought over