You dont really want fine sand with Oscars anyway, it gets dirty easily as Oscars are very messy fish. Gravel would be fine, but being Oscars, yes they will re-decorate as they see fit and each time you put it back, they will move it again. Bare bottomed are easier to keep clean with large messy fish, however, some dont like it, as they dont like the glare from the glass, particualary with the lights on.
If you have a lid on, I cant see how they could spit any gravel onto the floor, but they will move it around as said. With Oscars you really need a good fitting lid, they do like to jump and can often get out the tank, which is the last thing you want, especially if your not there to keep an eye on them, they can injure themselves or worse, be left dying on the floor. Whist they will still jump even with a lid on, at least they just give themselves a bit of a shock, not usually anything more serious, although Ive lost an Oscar due to his diving for the top and causing himself a fatal injury.
Plants, unless very well rooted, will be dug and pulled up, so plants anchored to wood or stones are better, personally I use good quality fake plants in my cichlid tanks, one so they cant be eaten and two, because no matter how hard they try, they cant be chewed up and spat around the tank, whist they can still move them, at least I dont find pieces floating around
