Preheat oven to 375°F (190°C).
In a skillet, heat olive oil over medium heat. Add garlic and sauté for 1 minute.
Add mushrooms and cook until softened, about 5-7 minutes. Stir in spinach and cook until wilted. Remove from heat and mix in cream cheese, parmesan, oregano, salt, and pepper.
Cut a pocket into each chicken breast. Stuff with the mushroom-spinach mixture and secure with toothpicks or kitchen twine.
Heat olive oil in a skillet and sear chicken on both sides until golden, about 3-4 minutes per side.
Transfer to the oven and bake for 20 minutes or until the chicken reaches 165°F (75°C) internally.
Let rest for 5 minutes before serving.