Either or both fish mentioned will readily spoawn in aquaria, and both lay the eggs on surfaces. A photo would help, but I would be fairly certain this is what you are seeing. Snail eggs appearing without snails is not possible; the eggs might come in on plants, but they would not be appearing on the glass without snails to lay them. Plus they are considerably smaller, usually invisible except for the blob of what looks like gelatin. The eggs of corys and otos are similar, about 1/16 inch, and white initially then darkeneing as they develop.
Corys will lay their eggs randomly on the glass, on plant leaves (upper or lower side), even on filter stems. Usually they will be laid as one, two, three or more in one spot, then others elsewhere. The female deposits each fertilized egg individually, after having cleaned the spot as she holds the fertilized egg in a pouch formed by her two pelvin fins; the male(s) will usually be with her. Spawning often occurs in very early morning before the tank lights go on, so may go unnoticed untill you see eggs.
Otos lay very similar-looking eggs, and reports indicate they spawn in much the same manner. I have had oto fry appear in some of my tanks, though I've never witnessed the actual spawning or eggs. I have had and have seen corys spawning.