1In food processor, place onion, olives, garlic and oregano. Cover; process with quick on-and-off motions, scraping bowl frequently, until mixture is finely pureed. Add olive oil; process with quick on-and-off motions until well mixed.
2Spread about 1 tablespoon olive mixture on 1 side of each bread slice. Top 4 slices with roast beef, cheese and tomato. Add remaining bread, olive side down. Cut each sandwich in half.