Depends which Order of Merit you're talking about. The Academy has Academic Order of Merit, Military Order of Merit, and Overall Order of Merit. Academic Order of Merit is simply the academic standing within the class determined by GPA (or QPR as we call it). Military Order of Merit is this mysterious number spit out by an algorithm that takes into account military class grades, professional competency, conduct, PRT, etc. Overall Order of Merit is determined by adding AOM and MOM and then ranking from least(top of class) to greatest(bottom of class).
Your academic OOM is important should you later want to attend grad school. Overall OOM used to be important in service selection b/c that was the order in which you selected. Now that the process is different, it still matters but not quite as much.
Thanks .....so the overall aggregate determines your actual class rank, not just grades alone? Also, how is Conduct and Military performance scored? I'm troubled when folks say the scoring of this is "mysterious".
It’s not mysterious. There is a Midshipmen Instruction that covers this. If you look around on Google you will find it. There are also some older threads on this if you search and might find the link there.