June 13, 2006 -- YOU'D think Oprah Winfrey would spend the night before the Tonys relaxing. But instead, the "Color Purple" producer hit Island Def Jam chairman L.A. Reid's 50th birthday party at Nobu 57 with her best pal, Gayle King, and partied until 2:30 a.m.

In what many called "the party of the year," Reid and his wife, Erica, rented out the whole restaurant Saturday night and celebrated with the music world's A-listers until 4 a.m.

To the surprise of many, Reid's former rival, Sony BMG honcho Clive Davis, showed up and said nice things about him. It's a far cry from 2000, when Davis was ousted as Arista head and replaced by Reid.

Three years later, when Reid himself was dumped from Arista, there were whispers Davis had orchestrated his demise. But Saturday, they looked like old friends.

Also there was Jay-Z, looking so slim some started whispering about a "Def Jam Diet." One partier noted, "L.A. has lost a lot of weight. So has Jay, [Jay's girlfriend] Beyonc