Heat a large non-stick skillet over medium heat and lightly brush with half of the olive oil.
Place one tortilla in the skillet and sprinkle 1/4 cup of mozzarella cheese over the entire surface.
Layer half of the feta, sun-dried tomatoes, olives, red onion, and spinach over one half of the tortilla.
Sprinkle another 1/4 cup of mozzarella over the fillings and season with half of the dried oregano.
Fold the tortilla in half to create a semi-circle and press down gently with a spatula.
Cook for 3 minutes until the bottom is golden brown and crispy.
Carefully flip the quesadilla and cook for an additional 2 to 3 minutes until the cheese is fully melted and the second side is browned.
Remove from heat and repeat the process with the remaining ingredients and olive oil.
Let the quesadillas rest for 1 minute before slicing into wedges.