Mischa Barton

Mischa Barton gave an impromptu performance of Britney Spears' hit song 'Oops!... I Did It Again' during a night out partying. The former 'O.C.' actress was enjoying an evening out in London on Tuesday, when she decided to treat revellers at the trendy Paper nightclub to some of her singing. Mischa, 20, jumped on stage and launched into the famous track, and even treated clubbers to a sexy dance routine. One onlooker said: "She got on stage clutching a microphone and a bottle of champagne. She seemed a bit drunk, but her singing was good. She certainly can hold a note."Mischa isn't the only young star who has been enjoying London's nightlife this week.Christina Aguilera racked up a massive $4000. bar bill while partying in the UK capital on Tuesday. The 'Beautiful' singer wasted no time getting into the party spirit at Kabarets Prophecy club, and she was later seen being carried out of the venue by her bodyguards after apparently having too much champagne. However, the 25-year-old star's unsteady exit was later blamed on exhaustion. A source said: "Not only is she very little but she was quite tired because she's in the middle of her tour. She only sipped a few glasses of Veuve Clicquot champagne and one vodka shot. Her party still ran up a pretty sizable bar bill, though."