Over supply and over production are the same thing. I personally prefer OP simply because IMO it is more accurate to talk about milk production than milk supply. A mom produces milk for her baby the entire time she is lactating. Assuming all is going normally, she can and will produce more or less as time goes on as is needed. She does not have a finite supply of milk.
Fast letdown (OALD) often occurs when a mom is overproducing. Most often, but not always, they are connected. The terms are often used interchangeably but it is true they are not the same thing. One describes a mother making more milk than baby needs, and the other describes milk flow that is so great, baby is finding it difficult to nurse or it is causing other issues.