well, i had a big apple snail w. my 10" oscar and he tries to kill it, sucks it up and spits it at the big rock cave or the glass in my tank, i found they didnt mix but it really depends on the personallity of your fish, (youll probably hear that alot)
Well I had 2 2inch ramhorn snails in with my dempsey which bred a ton before I had my dempsey. Well, now I do not have any snail problems. In fact I do not have any snails, but my dempsy is much smaller and very very bold. He is only about 3.5-4" and killed our 5 inch pleco.
i had a ramshorn (or whatecer those snails are that come in on plants and take over a tank) grow to about 2in in what seemed like overnight -- i went to show my mother the huge snail crawling up the glass, and as so as she came over to look at my clown knife swam up and ate the thing whole in one bite -- of course she disgusted but i thought it was hilarious -- i little off the topic but it was a funny story i wanted to share
i would say that an oscar that big would probably savor the escargo delicacy that is an apple snail