Pat the chicken dry with paper towels and season with salt and pepper. Heat the oil in a 12-inch skillet over medium-high heat until just smoking. Brown half of the chicken on both sides, 5 to 8 minutes per side, reducing the heat if the pan begins to scorch. Transfer the chicken to a plate, leaving the fat in the pan. When cool enough to handle, remove the browned skin from the thighs and discard. Transfer the chicken to the slow cooker. Return the pan with the fat to medium-high heat and repeat with the remaining half of the chicken.