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Oprah partied like it was 2004 ... invoking her most famous catchphrase when she delivered trays upon trays of pizza to surprise "Project Runway" star Michael Costello.

It all went down in downtown L.A. at the fashion designer's store where she showed up with servers and 14 pepperoni pizzas in tow. Michael was celebrating the 1-year anniversary of his joint on Monday, but had ZERO clue about what was gonna happen next.

With everyone gathered around, in came Oprah ... who earlier this month held an "O, That's Good!" pizza contest. Michael just so happened to enter the contest and, yes, he was one of the winners.

Cool moment is when Michael peels off a pepperoni and feeds it to Oprah, who congratulated Michael. They sipped on some bubbly but not before Michael dropped the obligatory "You get a slice! You get a slice!" reference.