It is possible to receive an LOA post nomination if other areas of your application have not been finalized - CFA/DODMERB (as examples). The LOA usually identifies a list of conditions that must be met to convert the LOA into an appointment.
If you have been green lighted by DoDMERB and have passed the CFA and obtained a nomination and are good on the academics, you are more than likely just waiting on the TWE/BFE vs an LOA.
I think last year there were some LOA's given very late in the cycle as candidates were waiting for medical waivers.
