Heat another pot over med-high heat and add olive oil. Add the diced onion and cook until onion is softened and translucent. Reduce heat to medium and add the tomato sauce, oregano, salt, and pepper. Simmer for 5 minutes, stirring occasionally.
Cook the tortellini according to the package directions and drain once cooked.
To layer the dish, first add sauce to an 8 ½ x 11 casserole dish. Add the tortellini on top, and stir to combine. Tear the mozzarella cheese and place over the top.
Broil until the cheese is melted and bubbly.
Remove from oven and top with fresh basil if desired.