O'Donnell, 26, best known as Al Pacino's sidekick in Scent of a Woman and as Robin in the Batman films, has dated the 24-year-old Fentress for almost four years. She's the kid sister of his Boston College roommate, Andrew. In fact, it was Andrew who introduced the pair.
Last December, the fresh-scrubbed actor discussed his wedding plans with E! Online. "I want a big wedding. I want to have all my friends and family there," O'Donnell said. "I'm not real worried about the press and how they're going to deal with it. They're going to get pictures of us leaving the church and going to the reception or whatever, that's fine, you know. There's nothing I can do about that. All I can do is make sure the reception stays private."
That reception will take place at the National Museum of Women in the Arts following the ceremony. No word on whether any of his Hollywood pals like George Clooney or Sandra Bullock will be there.
O'Donnell, who's appeared on People's most beautiful and best dressed lists, proposed to Fentress last year.
It will be the first marriage for both.