According to me OEM stands for "Original Equipment Manufacturer", and those are the folks like Dell and HP and Sony ..
I run a computer shop and i am also sell new and used auto parts of computer everyday. My new aftermarket parts are "OEM" they are similar like OEM, such as the fit, its style, and some 97% the quality as dealer products.And whenever you see OEM on a website such as ecstuning..etc., means that the parts fit as OEM but would be considered an OEM replacement part. The real OEM would come from the dealer . Honestly there is not really a difference in non-dealer purchased items, and I can back this up my dad's business has been open since 1986 and has never had any problems with our new parts. So it's just at your discretion.