The bash, held at her pal Guy Oseary's Los Angeles home, went on until 4 a.m. Celebrities were "wall to wall," a guest tells PEOPLE. "Everyone was dancing, mingling and eating."
Academy Award winners past and present let loose and dined on dishes such as Caesar salad, penne pasta with red sauce, rice and chicken in a curry sauce and potato knishes, served buffet style.
Best Actor winner Matthew McConaughey and his wife Camila were inseparable.
Julia Roberts and husband Danny Moder "were lovey dovey together," the partygoer adds, explaining that by the time Roberts arrived, she had ditched her Givenchy gown and "took all her glam off."
The show's host, Ellen DeGeneres, and wife Portia de Rossi were relaxed and "just hanging out," says the guest, while Penélope Cruz and Javier Bardem showed off pictures of their 7-month-old daughter Luna to friends.
It was also a destination for other A-list couples like Brad Pitt and Angelina Jolie, newly engaged Mila Kunis and Ashton Kutcher, Leonardo DiCaprio and Toni Garrn, Jennifer Lawrence and Nicholas Hoult, and Nicole Richie and Joel Madden.
Even Madonna's ex, Sean Penn, popped into the party with girlfriend Charlize Theron. The new couple were "hugging and very sweet together," says the source.
Also among the revelers: Bette Midler, Meryl Streep, Jamie Foxx, Miley Cyrus and Pink.
But it was Jared Leto and Lupita Nyong'o who brought the most coveted dates of the night: their 13 ½-inch-tall golden statuettes.
Madonna, whose kids also attended the party, changed her outfit at least twice and wore diamonds – lots of them: "Diamonds, diamonds and diamonds on both wrists, and a headband," says the source.
Additional reporting by AILI NAHAS