Preheat your oven to 375°F (190°C). Grease a baking dish with olive oil or non-stick spray.
In a skillet over medium heat, sauté the minced garlic in olive oil until fragrant, about 1 minute.
Add the chopped spinach to the skillet and cook until wilted, about 2-3 minutes. Season with salt, pepper, and dried oregano.
Remove the skillet from the heat and stir in the crumbled feta cheese until well combined.
Make a horizontal slit along the side of each chicken breast to create a pocket, being careful not to cut all the way through.
Stuff each chicken breast with the spinach and feta mixture, then secure the openings with toothpicks or tie with kitchen twine to keep the filling inside.
Season the stuffed chicken breasts with additional salt, pepper, and oregano if desired.
Place the stuffed chicken breasts in the prepared baking dish and bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through and no longer pink in the center.
Once cooked, remove the toothpicks or twine from the chicken before serving.