Prick each potato several times with the tip of a sharp knife. Combine the oil, salt, and pepper in a bowl and rub the potatoes all over with the oil mixture.
Arrange the potatoes in the insert of a 5- to 7-quart slow cooker. Cover and cook on high for 3 to 4 hours or on low for 7 to 8 hours.
Remove the potatoes from the slow cooker and serve, or serve from the cooker set on warm.