Pat chicken dry with a paper towel; rub with oil. Place lemon halves inside chicken cavity; tie ends of legs together with a string and tuck wing tips under.
Place chicken in a 5-quart oval slow-cooker. Sprinkle with seasonings. Cover and cook on high setting one hour; reduce heat to low setting and cook 4 1/2 hours or until a thermometer inserted into thigh registers 180°. Garnish, if desired.