Lightly brush 1 side of each slice of bread with oil. Place bread, oiled side down, on a clean work surface. Spread top of each slice with 1 tablespoon of the mayonnaise. Top 4 of the bread slices with roast beef, cheese, tomatoes and red onion. Top each with second slice of bread, oiled side up.
Place 2 sandwiches at a time on the preheated panini maker. Cook 3 to 5 minutes or until bread is crisp and dark golden brown. Cut each sandwich in half. Serve with au jus for dipping.