OEM manufacturer is one of those "Pop Up" company when US is involved in both war in 2000s. The Army needed a lot of parts to maintain their small arms stock, people have a milling and lathing machine would start making these part and bid for government contract. Leaving the big guy (KAC, Colt et el) focus on providing full service and support.
A lot of these company started to make hand guard, lower or upper receiver, then when they started to have excess stock, they will move on to a complete product. That's what OEM stand for. The quality of firearms is usually shit that's why they seldom have any customer, literally they are people who had Title 2 FFL with machine tool.