Place a large non-stick skillet over medium heat and add the olive oil.
Lay one tortilla flat in the skillet.
Distribute half of the mozzarella cheese evenly across the tortilla surface.
Layer the chopped spinach, crumbled feta, sun-dried tomatoes, and red onions over the cheese.
Sprinkle with dried oregano and top with the remaining mozzarella cheese.
Place the second tortilla on top and press down firmly with a spatula.
Cook for 3 to 4 minutes until the bottom tortilla is crisp and golden brown.
Flip the quesadilla carefully and cook for an additional 3 minutes until the cheese is fully melted.
Remove from heat and let rest for 60 seconds before slicing into wedges.