Short for Original Equipment Manufacturer.
In many industries, Original Equipment Manfacturers or OEMs supply components or assemblies which are incorporated into the final product, and their name (and no references to it) often does not appear on the product either.
The auto industry is a good example. Items like tires, wheels, headlights, etc., are not made by the manufacturer of the car. The tires and headlights can also be bought on the aftermarket, from a number of well known companies, many being the same brands found on a new car. Mechanical components may be made by (for example) TRW, but can be made by others too. Some parts, although sourced from an OEM, can only be purchased from the car dealer. Items like tires, headlights, filters, etc., with store brand names are usually made (OEMed) by a major maker of those products, to the buyer's specifications and price.
This gives the manufacturer the ability to purchase parts, from bearings to complete dashboard assemblies, from several sources.
Other companies may purchase a complete item, such as a television, from another large well-known manufacturer, with some cosmetic changes and their brand applied. Companies such as RCA did not make their branded VHS machines, they bought them from companies such as Panasonic (Matsushita Industrial Electric Co.). Many large manufacturers in many industries manufacture products not only with their own brands, but for other companies and retail chains.
In DCC, the same is true. Some of the DCC systems out there are either rebranded or customized versions bought from an OEM, such as Roco. They could also include components supplied by others. NCE started out supplying components, and software which would be incorporated into a finished product. The Wangrow System One was designed by NCE and manufactured by contractors working for Wangrow Electronics.
There are OEM decoders, which are stripped down or customized versions, sold exclusively to another manufacturer. QSI and Soundtraxx supply decoders for Atlas, Athearn, BLI and others. Another well known supplier of decoders on an OEM basis was MRC.
Another term is Original Contract Manufacturer. While not a common practice in the DCC world, in the electronics industry it is. For example, laptops. Several brands, while cosmetically different, internally are identical, made at the same factory. The OCM designs the device to set specifications and cost targets, other companies then buy them with some cosmetic differences and their brands applied. The same would also apply to televisions, DVD players, and other electronic devices. There are many companies which do not manufacture anything, they deal with OCMs for their branded products. Some companies don't even sell the products with their brands, they simply licence them (the brands and trademarks) to others. (The opposite is also true, some companies refuse to sell their products with another brand on them.)