In a large skillet, cook the beef, green pepper and onion over medium heat until meat is no longer pink; drain. Stir in the pizza sauce, pepperoni and oregano.
Spoon about 1/2 cup beef mixture off-center on each tortilla; top with a piece of string cheese. Fold one side of tortilla over filling and roll up from the opposite side. Place seam side down on an ungreased baking sheet. Bake at 350° for 10 minutes or until heated through and cheese is melted.