Original equipment manufacturer (OEM) is a term used when one company makes a part or subsystem that is used in another company's end product. The term is used in several ways, each of which is clear within a context. The term sometimes refers to a part or subassembly maker, sometimes to a final assembly maker, and sometimes to a mental category comprising those two in contrast to all other third-party makers of parts or subassemblies from the aftermarket.
In the first usage, the OEM is the company that makes a part that is marketed by another company, typically as a component of the second company's own product.
In the second usage, OEM refers to companies like value-added resellers, which are the second manufacturer in the definition above.
In the third usage, OEM is a mental category for all of the makers involved when a final assembly was first built (originally equipped)—in contrast to whoever made aftermarket parts that were installed later.