Preheat the oven to 375°F (190°C).
Place each chicken breast between two sheets of plastic wrap. Use a meat mallet or rolling pin to gently pound the chicken until it is about 1/2 inch thick. Remove the plastic wrap and season the chicken with salt and pepper.
In a mixing bowl, combine the chopped spinach, ricotta cheese, mozzarella cheese, Parmesan cheese, sun-dried tomatoes, garlic, and oregano. Mix well until fully combined.
Spread an even layer of the cheese and spinach mixture over each chicken breast, leaving a small border around the edges.
Starting at one end, carefully roll each chicken breast tightly into a roulade. Secure the ends with toothpicks or tie with kitchen twine.
Heat olive oil in a skillet over medium heat. Sear each roulade for 2-3 minutes on each side until golden brown.
Transfer the roulades to a baking dish and bake in the preheated oven for 20-25 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
Once baked, remove from the oven and let rest for 5 minutes. Remove toothpicks or twine before slicing.
Serve warm, garnished with fresh basil leaves if desired.