I was 2 or 3 days over due with my 6th, I took castor oil and with in an hour or so I was running to the restroom. It was about 6:00pm when I took it and my son was born at almost midnight.
With my 8th baby, I was 38 weeks and a few days. My mw and I knew he was going to be big, and I had had 2 shoulder dystocias (with my 2 previous births) because of GD. We were trying to get things started so that I could have my homebirth and not have to go to the hospital. I took the castor oil, more than I did with number 6, and NOTHING. I mean ABSOLUTELY NOTHING, I didn't even have to use the bathroom once. We tried other things, nipple stimulation, sex, stripping membranes, etc nothing worked. I ended up at 39 weeks at the hospital for an induction, no shoulder dystocia, but thick meconium that my mw would have transfered for anyways. I don't regret trying to get things started because I very much wanted my homebirth, but it wasn't meant to be.
So, IMO, if your body is ready it will probably work, if your body isn't ready then it probably won't.