Castor Oil is a bit unusual in that once enough is given, more does not produce more powerful laxative effects.
So, giving even 8 ounces won't make someone poo more strongly than a lesser dose. The larger dose is harder to take and keep down.
The only effect will be that you poo out a bunch of undigested oil.
When used as a punishment, it has several aspects:
The oil tastes and feels nasty. While it can be given in disguised ways, as a punishment the plain oil is the choice.
If someone is given the oil with an audience, there is the embarrassment factor. Everyone KNOWS what is going to happen.
The oil may cause nausea, but the recipient has to struggle to keep it down, and the taste repeats.
The recipient knows what is about to happen when the cramps start, and the punishment can be enhanced by denying bathroom access.