The musicians are performing their Oscar-nominated songs during the awards ceremony at the Samuel Goldwyn Theater in Beverly Hills, Calif., Fox News reported.
"All I care about is the fun," Williams said to actress Kate Hudson, who was also on hand to rehearse her part in the night's festivities.
Williams' "Happy" is nominated for the animated "Despicable Me 2;" O's "The Moon Song" is nominated for "Her;" Menzel's "Let It Go" is nominated for "Frozen;" and U2's "Ordinary Love" is nominated for "Mandela: Long Walk to Freedom."
The Academy Awards air 7 p.m. EST Sunday on ABC, Oscar.com, ABC.com, WatchABC.com and the ABC Watch App.
If you feel like doing a little preparation yourself, watch the official video for Williams' "Happy" below: