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.