It depends on what era you're referring to. Oprah hasn't always been as immensely popular as she is right now, and if she "came out" right this instant and started pushing for same-sex marriage, it probably wouldn't have quite the same impact it would have earlier because (a) it's generally assumed she is lesbian these days anyway, and (b) I think we're rapidly on our way to SSM as it is, without any PoDs needed.
If we want to go with "star power" as the means to softening up mainstream opposition to SSM, with the idea that it's fully in effect by this date, I would say we should go with Tom Cruise at his heyday in the '90s (he has always been rumored to be gay, anyway), Julia Roberts, Mel Gibson, etc. A "dark horse" is the rapper Eminem, who (especially in his earlier career) made near-constant references to homosexuality in his music and got a lot of notoriety for it. He would likely sway a lot of youths.