Dustin Diamond Busted for Oxycodone

Dustin Diamond, from "Saved by the Bell," violated his probation by failing a drug test ... TMZ has learned.

According to docs ... Screech's urine tested positive for oxy during a test administered last month. Diamond told his parole officer he'd recently taken "a painkiller" for a toothache -- but he also admitted to not knowing what kind it was ... because it was an old prescription.

That's his story, he stuck to it ... and that's why he got thrown in jail for the violation. Diamond was released after 2 days behind bars.

He told cops he's "cleaned out" his medicine cabinet.