OEM is system builders, It's cheaper because unlike a full license you cant transfer it from one pc to another, where PC is defined by Microsoft to be, one motherboard to another.
However in the event of motherboard failure you contact microsoft and get it reactivated, and people use this, claiming the motherboard failed to get it activated on a new PC anyway. So really, the OEM is the best value, especially if your not intending to change your motherboard for a long time.
Note that you can change your CPU, GPU, Storage and RAM without trouble, although only 3 of them at one time I believe.