First clarify what do you mean by "solder mask". The OBTACLE you draw or the PAINT on the board. It is a clasical use of word confusion.
I am going to name SOLDER MASK the obtacle you draw in Orcad.
Suppose you DO NOT want paint on the whole bottom layer (RF board a must), you want only finished metal exposed.
In this case you have to copy the board outline on the soldermask layer.
In conclusion, you have to have an obtacle on the SM layer over the area where you do not want paint (or you want only metal exposed).