How deep would the nitrogen and oxygen ocean be if cooled to liquid?
I'm not sure it would be entirely liquefiable as the atmosphere gets thinner and thinner, but if we ignore that, then it's a simple matter of calculating density and ocean cover.

78% Nitrogen, 0.807 g/mL  Source:  Liquid Nitrogen

21% Oxygen, 1.141 g/mL  Source: Liquid Oxygen

Ignoring the 1% of trace gases, average density of liquefied air, 0.87 g/mL, or 87% the density of water.

Mass of air, 1 ATM is equivalent to 33.9 feet of water.   Source:  link

and the Earth is 71% covered by oceans.   From there it's just math.  33.9*(1/.87)*(1/.71) = 54.88 feet.    That's quite a bit less than the oceans would rise by freezing solid (which might not happen given volcanism and warmth coming from the earth, but if even half of the average depth of 14,000 feet of oceans froze, a 9% increase in volume, we'd be looking at some 600 feet of sea level rise by freezing.  

Colder temperatures might cause the liquid nitrogen/oxygen to contract a bit further but that's a ballpark answer - 54 feet, 10 1/2 inches.   Some of the air might dissolve into the oceans prior to the oceans turning to ice, cause colder water can hold more gas, so the actual answer might be a little bit less than that.