Mischa Barton is in fine form during the holidays, thanks to a new program of healthy eating and running, she’s telling friends. The actress, whose fluctuating weight and questionable fashion choices have been well documented since her breakout role in the popular defunct series “The O.C.,” flaunted a slimmer, toned shape in a skimpy black bikini while hitting the beach over the holidays in Miami. Barton has been telling friends that her key to staying slim is “eating really healthy, keeping a mostly vegan diet and running.” Barton was spotted at Prime 112 Thursday night having a light dinner with friends and later at the W South Beach for drinks.