Does that clear it up? Probably not. Acronyms are funny like that. The real definition is easy to understand. OEM is usualy used when referring to auto parts. If a part is called OEM, it was made by the manufacturer to be used in your car when it was new, and can be bought later to replace a broken part. For instance, if you drive a Ford, an OEM window crank was built by Ford to be used in your car. The window crank you see hanging on the rack in the auto parts store is not an OEM part, because it was manufactured by somebody else and only used to replace the window crank Ford installed on the assembly line.
Since nothing is as simple as it could be, there's more. Sometimes Ford hires an outside company to produce all of their window cranks for that year. Let's say they hire Acme Parts to make them. In this case, Acme is the OEM supplier for window cranks, making them official Ford parts since they were installed on the assembly line. So they can sell Ford window cranks later, under the Acme Parts name, and still call them OEM window cranks.