OEM is an acronym for Original Equipment Manufacturer. As its name suggests, OEMs are companies that manufacture products for other companies that will typically re-brand the products and sell them. This is very common practice where companies with strong brand recognition would source products from other companies that either don’t have the same brand recognition or simply prefer to do business with other businesses only.
For example, power supplies are a product that companies commonly source from OEM manufacturers. While EVGA has an entire lineup of power supply offerings, all of these power supplies are manufactured by other companies such as Seasonic, Super Flower, and FSP. As such, Seasonic, Super Flower, and FSP can be referred to as OEM companies even though they also sell products under their own brand.