OEM means just that. Original Equipment Manufacturer. As the Doctor pointed out, that can mean different things to different people. However, the accepted definition in industry (not just the Auto industry, but industry at large) is that OEM means a part made by the same company and to the same spec as the original.
For instance, if your car came from the factory with BOSCH spark plugs and wires, then only BOSCH plugs and wires of the exact same model/type are OEM replacements. Period. Anything else is "aftermarket".
There are many brands of plugs and wires (to continue my example). Most manufacturers will make a set of wires and plugs that meet every specification for the original equipment that came from the factory, but these are still not OEM. They are called "OEM SPEC". This includes the $1 chinese parts.
Stock purists will insist on OEM parts only and will not accept "knock-off" parts, even when they meet spec.
guess it depends on type of industry, the use I had was at at a much higher level in corporate world where management/marketing decided to use their own product [eg at Carrier UTC we used our own "Building Automation System" at MIT and DisneyWorld] but I was at some heated meetings where some of the marketing dicks wanted to "use OEM" eg from Honeywell, which was a much inferior system
so for 2 years we had an arrangement with Honeywell to rebadge their OEM product as Carrier 7000 or whatever but the shit hit the fan, we lost customers and went back to the better product and no OEMs
but I can see where OEM could be used differently in auto industry where we are talking a mass production product
but bottom line is I was using it in context of MY interp when I was explaining the [possible] ISO connections on car radio so good to know there are 2 meanings and maybe I will not use the word again but just say made in China