Heat olive oil in a skillet over medium heat and brown chicken thighs on both sides, about 3-4 minutes per side.
Transfer browned chicken to the slow cooker. Add diced tomatoes, bell peppers, onions, garlic, white wine, oregano, red pepper flakes, salt, and black pepper.
Cover and cook on low for 4 hours until chicken is tender and cooked through.
Remove chicken and serve hot with sauce spooned over top.
Notes
Serve with crusty bread or over pasta for a complete meal.