OEM = Original Equipment Manufacturer this term is misused by many. It means it is a Genuine GM fender.
The best OEM are only made by GM but they discontinued them. The ones on eBay are 99% aftermarket. They are thiner but will work for most people.
After market is what most places sale and they are around $80.00 some sell higher. Autozone and most places can get them. LMCtruck.com, classicduty.com