Heat a large skillet or griddle over medium heat.
In a bowl, toss together chopped spinach, feta, sun-dried tomatoes, olives, and oregano.
Place a tortilla on a flat surface and sprinkle a layer of mozzarella on one half.
Top the mozzarella with the spinach and feta mixture, then add another light layer of mozzarella.
Fold the tortilla in half to close.
Lightly brush the skillet with olive oil.
Place the quesadilla in the skillet and cook for 2 to 3 minutes per side until the tortilla is golden brown and the cheese is melted.
Repeat for remaining tortillas.
Slice into wedges and serve.