Pure nitrogen and OFN, are the same thing, you purchase OFN as a method of purity.
Triple evacuation, if you have free moisture (water droplets), when you go into a deep vac, you go below the triple point. Some of the water vapourize and some will turn into ICE. Continue vacuum, the ice will sublime (like dry ice "solid CO2") The density of the vapour can be many 1000M3 per Kg, so your vac pump has little or no effect. You now add nitrogen basically to add energy to the system, raising the boiling point above the tripple point.