Join Date: May 2017
Location: Brisbane Australia
Air is 78.09% Nitrogen, so unless they suck all of the air out of your tyres before they fill them up with nitrogen, you'll still have a fair bit of O2 in there. Nitrogen is a gas and is still affected by changes in temperature, almost to the same degree (pun intended) as air.
There are some benefits of using Nitrogen: Nitrogen is less likely to escape through microscopic pores in the tyres (meaning that the pressure over time will stay more consistent), and nitrogen doesn't contain humidity that can be present in compressed air.
However, it would only make sense to fill up with Nitrogen if it costs less than $1 per tyre.