OEM is not an upgrade, it's a full version. OEM software is what Dell, HP, Gateway, etc buy. OEM pricing is one way MS steps on home builders: Your "not supposed to" be able to buy this stuff. But there's a loophole: Small system builders.
You know, mom & pop shops. They can buy OEM software from distributors such as Newegg, to include in new systems they sell. And you become a "mom & pop shop" with one customer, yourself. The restrictions normally are that it has to be sold with "qualifying hardware".
Unlike the full retail version, you can't use OEM software for your choice of clean instal or upgrade, it's clean instal only. On one hand you have upgrades, on the other hand you have OEM full versions, and retail full versions have both hands.