Put the olive oil, lemon juice, garlic, and oregano in a large measuring cup. Whisk well. Salt and pepper both sides of the thighs. Pour about a third of the olive oil-lemon juice mixture into a gallon heavy-duty sealable plastic food storage bag. Add 4 chicken thighs. Pour in another third of the mixture, then add the remaining chicken and the remaining mixture. Seal and turn the bag to combine. Place in a baking pan and refrigerate for at least 2 hours. Heat a grill to medium, 400°F. Oil the grates. Place the chicken smooth side down on the grill and cook until browned, 7 to 8 minutes. With tongs, flip the chicken over and grill until the internal temperature registers around 170°F on an instant-read thermometer inserted into the thickest part, 5 to 6 minutes longer.  Transfer to a serving platter and top each thigh with a small pat of butter, which will quickly melt. Serve with lemon wedges.