The tone for an Oscar theme party starts when you write up the invitations. Use invitations with a movie theme--spools of film or images of spotlights on a premiere night--or try email invitations from places like Evite, which let you pick specific Oscar imagery. When your guests arrive, decorate your home with little signs of movie magic. Ask your local video store if you can purchase any used movie posters (they often sell them for just a dollar or two), or get a length of red cloth at a local arts and crafts store to use as a red carpet in front of your home. Some people like dressing up for Oscar parties--and if you've got the tux or formal dress, go for it--but if you want to keep things casual, just wear a T-shirt or jacket with a movie image on it.
Food can be as simple or as elaborate as you wish. See about getting munchies with a movie theme: hot popcorn, soda in movie theater cups and concessions-stand candy. If you're looking for something more in-depth, contact a bakery to see if it can make an Oscar-shaped cake, or a pizza parlor to see if it can cook a pizza decorated like a movie reel. In some cases, you can tailor your snacks to match those of a particular film. For instance, the 2008 Best Picture winner, "Slumdog Millionaire," was set in India ... giving people a good excuse to serve Indian food.
Oscar parties are much more fun when you make a game out of the proceedings. Hand out ballots and ask everyone to predict who's going to win each award. Keep a running tally throughout the night, then give out a door prize to whoever gets the most right. Many party stores sell small faux Oscar statues, but you can make it anything you want. (A DVD of a really bad movie is a great idea, especially if it features an Oscar-winning actor. You'd be surprised the sort of hilariously awful films containing great actors.) Other party favors can go to everyone who came: bags of movie candy, cheap sunglasses purchased at a local thrift store or maybe a burned CD with the tunes nominated for that year's Best Song category.