No question it’s serious between “Ocean’s Eleven” star Matt Damon and sweetie (or should I say . . . fiancee?) Odessa Whitmire, assistant to Matt’s pal Ben Affleck! Matt and Odessa slipped into Cartier in BevHills and spent half an hour checking out engagement rings and settings — then left empty-handed! Damon’s rep keeps insisting they’re not engaged, but . . . stay tuned!