I would be VERY careful using a piece of wood you don't know the history of. trust me, i just lost my favorite angelfish pair around New Years for the exact reason. I bought a piece of something that turned out to be rotten, and a week later it killed the tank.
What I would do if I were you is boil the wood for around 2-3 hours if possible, dry irt really well and then set up the tank, but give it at least 2 to 4 weeks without fish to make sure the wood is safe, and to give it a chance to "season". If nothing happens, then go for it and get an oscar. Again, take it from me, i just made a horrible
driftwood mistake, don't make the same one I did.