will oscars the fish be ok with a?

I don't recommend it, even in a 500g really. You need to cater for the RES which means it needs land and heat lamps to bask under. Oscars are territorial and curious, if it gets too close to a large RES, you'll end up with half an Oscar.

Turtles are best in species set ups, not mixed with fish!

If you're going for a 500g, sign up to this site:

http://www.monsterfishkeepers.com/

Plenty of owners of tanks over 1,000 on there (and one 50,000).

Probably not, unless they are in a truly huge tank.

Turtles will eat any fish they manage to catch, and the Oscar could also make life pretty miserable for the turtle.

Also, the turtle needs a part land/part water tank, which will cut into the water volume available to the Oscar. Since an Oscar gets to be about 14 inches long, and heavy for that length, it needs at least a 75-gallon tank-- and that's 75 gallons or more of actual water in the tank.