Mischa Barton's Home -- 'The O.C.' Star Facing Foreclosure in the 90210

"The O.C." star Mischa Barton could lose her massive Beverly Hills mansion in foreclosure ... if she doesn't cough up $100K in back mortgage payments QUICK.

Barton just got slapped with a default notice -- obtained by TMZ -- informing her she's skipped more than 100 grand in payments ... triggering the foreclosure process.

The crib is awesome -- 7 bedrooms and 11 baths all on 1.2 acres. She bought it for $6.4M in 2005 -- when "The O.C." was a big deal, and took out a loan for $4.2 million.

The show went off the air in 2007 -- and Mischa's been trying to unload the house since 2010 ... when she listed it for 8.695M, but no one bought it. She re-listed in 2011, and again ... no takers.

Her last ditch effort was attempting to lease it for $35K/month last year ... but that move appears to be way too little too late.

Mischa's been acting in some small stuff lately, playing Ms. Monica in the TV movie, "Gutsy Frog." She also did an appearance at an Atlantic City club.