"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Oldsmobile Omega, if you buy accessories for your car that are not actually made by Oldsmobile, then those are termed "aftermarket". Many people use the term "Omega aftermarket accessories" specifically when referring to custom accessories for the Omega, like performance accessories or custom appearance accessories. But even if you just replace your Omega brake pads with OEM-grade factory-style brake pads that are not made by Oldsmobile, those too are aftermarket Omega accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".