Heat vegetable oil in a large heavy skillet over medium heat; sauté onion until tender. Add ground beef to skillet; brown, breaking up as it cooks. Drain off excess grease and return skillet to heat. Add tomato sauce, oregano, and salt. Simmer, uncovered, for 10 minutes.
Spoon meat mixture over buttered rolls; top with cheese triangles then arrange sliced green olives or pickle slices over the cheese. Broil for 3 minutes, about 4 inches from heat source, or until cheese begins to melt.
Makes 8 open face sandwiches.