The Story of Ohhhh: Once upon a time there was an utterly irresistible dessert concocted in the magical kitchen of the Dessert Gallery Bakery & Café[3200 Kirby, (713)522-9999]. The pastry elves called it "The Big O," but it's not what you're thinking: This cake's a double-layered stack of creamy mousse filling, one dark and one white chocolate, both plentifully studded with crumbled Oreo cookies. Oh, that "O"! The whole is thickly iced, top to bottom, with fudgy chocolate set shiny. More Oreos delectably decorate the top, these dressed to the nines in tiny chocolate icing tuxedos. Ahhh. By the piece, $4.95; by the cake, $37.50.