Cook and stir ground beef and onion in large skillet over medium-high heat until beef loses pink color.
Pour off drippings.
Sprinkle oregano and salt over beef, stirring to combine.
Place tortillas on 2 large baking sheets.
Lightly brush surface of each tortilla with oil.
Bake in preheated 400°F oven 3 minutes.
Divide beef mixture evenly over tops of tortillas; divide tomato and desired topping over beef mixture.
Bake at 400°F 12 to 14 minutes, rearranging baking sheets halfway through cooking time.