Do you know what caused the oil canning? Don't jump in with a shrinking disc until you can determine the cause. Did the panel experience some body damage, or have you been welding on something? Different method of repair depending on the cause, so yes, to answer your question it can be cause by either too much metal or not enough.
I would suggest reading this first for the theory....
http://www.67-72chevytrucks.com/vboa....php?p=7305501
….and then read this thread in its entirety for a real world application, where welding was the cause... and planishing (stretching) was the corrective action.
http://www.spiuserforum.com/index.ph...l-canning.5502
It's hard for anyone to give you accurate repair advise tailored to your exact situation without more information about what you have. Pictures would help, a video of you cycling the oil can would be even better.