I've never seen an OML apply specifically to a unit, but to individuals. I can't speak for how ODAs conduct their OML's, but in my Support Company ours was based off of mission need/ requirement and then a soldier's "status" based on his rank, APFT scores, NCOER's, etc. It could also be a little subjective.
For example, the mechanics and the signal guys would need...we'll say 4 Jumpmasters per detachment for the sake of argument and each detachment currently has 2 qualified JM's. Within those two detachments the commanders would look at their individuals so an E-7 would usually carry more weight than an E-6 for the OML unless the E-7 declines the school (unheard of) or has something where he can't attend (medical profile, personal conflict, another school). Once the detachments figure out their OML's it would go to the HQ element and the company commander, first sergeant, and training NCO would figure out who is going next across the entire company.
As a detachment commander I never personally saw a formal "on paper" OML, it was usually "We have 3 slots for JM school. SIGDET gets two because they're short and the MID gets one. Who are you guys sending?"
Again, the ODAs may run theirs differently, even conventional units may do things differently than we did, but that is how our system worked. It isn't sexy, just "need" + "best candidate."