But the mass of the extracted oxygen exceeds the mass of the ammonia used, and we're only talking about 250 PSI to liquify anhydrous ammonia at room temperature

But the mass of the extracted oxygen exceeds the mass of the ammonia used, and we're only talking about 250 PSI to liquify anhydrous ammonia at room temperature. A large pressure vessel's mass is going to be negligible compared to the mass of the ammonia it holds. So, the same displacement ship traveling at the same speed can supply the production of more steel if you ship the ammonia to the iron ore instead of the other way around.