Mischa Barton Hosts Event ... BY HERSELF -- Casino Entourage: Thin to Win

Mischa Barton bucked the trend -- her entourage caused zero problems in an Atlantic City nightclub ... because TMZ has learned she rolled completely solo to host the event.

Barton was the celeb guest host for Pool After Dark last Saturday night at Harrah's Resort. A rep for the club tells us hosts usually have entourages 5-15 people deep, but the former "O.C." star thinned her crew down to a party of one.

Sounds like Mischa could've used a wingman -- we're told she stayed at the club until 3:00 AM, talked to no one, ordered a bottle of champagne and a bottle of tequila, and did shots ... by herself.

Barton did a court-ordered rehab stint in 2007 following a DUI, but has never claimed to be sober.

A source close to Barton tells us ... Mischa decided to attend last minute and opted to roll solo rather than miss the payday for hosting.