Nitrogen is used in aircraft for the same reasons we are seeing it put in cars..The Nitrogen Molecule is much larger than the Oxygen molecule..so therefore it leaks out less through natural attrition...Hence more stays in the tire longer.
The heat to expansion rate is slightly less than oxygen, but not enough to make any difference according to Boyles Law...
Nitrogen in the car tire is more of a sales hype than anything...
In aircraft tires, it is more important that Nitrogen is used because it is inert and will not sustain any kind of combustion or fire..And being dry type, by means of dehydration while it's compressed into tanks, makes it good to keep moisture out of the tires and forming the ice crystals as mention here below by terrainnewby...
For the record: Our Atmosphere contains a moist; 78% Nitrogen, 21% Oxygen and 1% misc. gases.