Place tortillas on a clean work surface and evenly distribute tuna, cheese, tomatoes, olives, and onion on one side of each tortilla.
Fold the other half of each tortilla over ingredients and set aside.
In a large skillet over medium heat, warm oil.
Add quesadillas and cook until golden brown on the bottom, 2-3 minutes.
Flip and cook 2-3 minutes more until golden brown and cheese is melted.
Transfer quesadillas to a cutting board, and cut each into 4 wedges.
