"O.C." wanna been Mischa Barton has decided to take a plea bargain in her DUI case -- clever girl that one.

A source tells TMZ that at her court date on Thursday she'll plead no contest to the DUI charge. In exchange, the marijuana charge will be dropped and the driving without a license rap will be reduced to an infraction.

Greasy's ex-GF will then have to serve three years of unsupervised probation, attend an alcohol education class and pay a fine.

A source tells us this is "fairly standard" for a first-time offender like Miss Mischa, who was arrested last December, adding that she's "learned her lesson."