When Mischa Barton got busted for DUI earlier this week, cops said they found some narcotics in the former O.C. stars' car after Mischey blew a .12 breathalizer.

Evidently those unnamed narcotics were marjuana and prescription pills.  TMZ cracked a source inside the West Hollywood police precinct who spilled the beans.  Their source also says that Barton admitted to smoking weed earlier that day.

Cops pulled Mischa over after they say she was "straddling two lanes of traffic and failed to signal when making a turn."

After being picked up on suspicion of DUI, possession of narcotics and driving on a suspended license... Mischa was booked and released on $10,000 bail.