OEM =original equipment manufacturer
OEM is what DELL and all the rest of them put into your PC. Today you can buy the same stuff they put in. I buy DVDRW for the PC that way. Cheaper to buy, but you don't get a box (depends who you get it from), no screws, no CD (depends on where you get it from)
Buying a HD, just be careful where you buy a OEM from and don't get factory rebuilt an etc..
I buy HDs Retail/Office Supply/Warehouse Clubs and prices have come down, call me old fashion...