You should not try to induce labor on your own before 40 weeks or without the assistance of a trained professional. Castor oil will not really induce labor- that's an old wives tale. It will however act as a laxative
Castor oil is very commonly used to induce the labor pains in post dated pregnancy. Castor oil stimulates the peristalsis and stimulates the uterus. It is a common experience to see the patient coming with labor pains very next day.