Gravel vaccing sand in the traditional sense is basically a no-no. Can't do it without sucking up virtually all the sand. You have to hover your vac or tube over the sand to pick up debris.
Sand could be done with an oscar no problem, but it would be a lot of effort and you'd probably want to syphon up the poo most days to keep the tank looking reasonable.
Gravel is good with fish which produce lots of big poo, as it tends to visually hide the poo somewhat. In one of my tanks I have 1-2mm gravel, which actually looks and acts much like sand, but you can also gravel vac it. It's a good compromise between the two.
Personally, with an oscar I would use a smooth 3-5mm gravel. They will enjoy digging in it. It won't hurt their mouths, and should be pretty easy to vac and maintain for you.
No substrate is another option, bit personally I don't like the look of that, and also it removes the enjoyment an oscar might have of digging in it.