It's just another way of saying "stock." The parts that come standard on anything: Cars, Computers, boats, planes, electric tools, ect. just to name a few.
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.
OEM+ stand for "Original Equipment Manufacturer +".
This term originally comes from the automotive world to describe the customization of a car in a subtle way, (mainly) using parts and accessories from the same manufacturer or the same group (Using Audi parts on a Volkswagen for example) . More generally it means customize a car very discreetly, with a close look to the original, playing on the details. In contrast of tuning and its extravagance.
_ Wow, never seen a VW Golf so clean! Does it come from the factory like this?
_ Well almost, it's OEM+. Just changed a few details to enhance the original look.