Hello JAYDESAI, I'm no laser expert, but possibly it is due to the differences of reactivity of the two gases. I would imagine that the oxygen is more reactive and thus you see more dross, whereas the nitrogen is not nearly as reactive or the reactivity is in a different form. A thought for consideration. Best regards, aevald

no laser expert myself either. The issue being here in cutting galvanized you are trying to counter the sacrificial zinc coating that has different properties, which reminds me I hope there is proper smoke extraction for the zinc fumes!

assist gases for lasers operate on different principles depending on gas primarily they are trying to expel the molten material by force. Or in the case of oxygen by force with rapid oxidation. My guess is that galvanized zinc coating is preventing the proper  oxidation of the base material hence the normal speed advantage of oxygen is lost or will not work altogether so either travel speed should be slowed or nitrogren should be used. 

our shop uses both oxygen and nitrogen, you can cut with oxygen but it generally gives an inferior edge.

That's my educated guess