Preheat the oven to 375°F (190°C).
Butterfly the chicken breasts by slicing them horizontally, but not all the way through.
Season the inside of the chicken with salt and pepper.
In a bowl, mix spinach, cream cheese, feta, garlic, and oregano.
Stuff each chicken breast with the filling mixture.
Secure the openings with toothpicks or kitchen twine.
Heat olive oil in a skillet over medium-high heat.
Sear the stuffed chicken breasts on both sides until golden brown, about 3-4 minutes per side.
Transfer the chicken to a baking dish and bake for 20-25 minutes until cooked through.
Let rest for 5 minutes before slicing and serving.