OEM simply mean Original Equipment Manufacturer.
As Popularly referred, OEMs are manufacturers/Producers who resell another company's product under their own name and branding..
OEM refers to the act of a company branding a product to its own name and offering its own warranty, support and licensing of the product.
The term is nothing but a misnomer because OEMs are not the original manufacturers..These manufacturers customize the original product.
For example, if Samsung makes a phone that will be marketed by itel, an "itel" label will get stuck on the front, but the OEM of the phone is Samsung.
You can also use the term as a verb, such as, "That itel phone over there is OEM'd by Samsung."
Hope it's a time well spent reading through?
Reference
Webopedia