In a small bowl, combine the olive oil, balsamic vinegar, garlic, oregano, sea salt, and black pepper.

In a 4- to 6-quart slow cooker, place chicken thighs. Pour the balsamic mixture over the chicken. Turn the chicken thighs to coat with the balsamic mixture. Add the butter on top of the chicken.

Cover and cook on LOW for 6 hours or until the or until a meat thermometer inserted in the thickest part of the thighs registers 165°F.

Serve topped with the parsley