I just bought a oscar from petco and it has some white milky stuff on its eyes and i was wondering what it is and what i can do about it??
Also what size tank should i put it in when it gets bigger??
That's a sign of illness. Take it back since there's a 15 days contract on live fish, and they will let you get another one, but not that a minimum tank size of 40 gallons is recommended if you are keeping an Oscar.
No matter what the size of Oscar now, it should be in a tank suitable for it's adult size now. Moving a small fish to a big tank is easier than moving a big fish to a big tank! I would say at least 65-75 gallons for a single Oscar.
Milky and cloudy eyes can be a sign of water quality issues and a bacterial infection. Treat with anti-bacterial medication.