Any opiate with the prefix oxy, hydro, etc in front of the name. Valuable because of the fact that they are not metabolized into morphine which makes the standard drug tests test for morphine in the urine. They can however be detected by extended drug tests if opiate use is suspected.
person one: "Dude, you're on probation, you should't take that oc."
person two: "Nah man, oxy's a prefix opiate, it'll be cool."