OEM stands for original equipment manufacturer that means the manufacturer of the the OEM boat props that usually come on your boat and the marine drive system on your boat. If you want a different boat prop or spare boat propeller you have a choice of either purchasing an OEM propeller or a prop made by an independent company that makes props to fit your drive unit. These aftermarket manufacturers have been around almost as long as the original equipment manufacturers have. There is a huge market for boat props because they are easily damaged and need to be repaired or replaced by most boaters at some point in time.
Pros and Cons of OEM Boat Props vs. Aftermarket Boat Propellers
OEM boat props are usually more expensive than an aftermarket equivalent. In many applications they do not perform any better or much better than some of the aftermarket boat propeller brands. The quality of most of the aftermarket boat props has improved over the years and are generally now excellent quality propellers. There is very little difference in the looks of either types of boat propellers and you can get most of the same sizes of the OEM boat props in the aftermarket brands both in aluminum and stainless steel. There are propeller makers of both types based in the US and there are companies based overseas.
There will always be boaters who will run only OEM boat props because they believe they are much better and there are boaters who are put off on the high prices of a lot of the OEM products and have found that the aftermarket stuff works just fine most of the time.