Original Equipment Manufacturer.
OEM software is what you get "bundled in" free when you buy a computer. It might include Windows, some complete older programs, "taster" versions of new programs, etc.
OEM stuff is unsupported and comes without manuals or any kind of help. It's a free gift, take it or leave it.
Buying second-hand OEM software can get you a real bargain, as long as you know enough to get by without the manual or support.
When I bought my new computer, they gave me five OEM programs free.