Nitrogen is used. It is an inert gas so high pressures and temperatures do not affect it as with air. It is also more compatible with rubber, and does not corrode like air containing oxygen and water vapor does. More importantly it will not support combustion or explosion as does air containing oxygen.
The FAA requires nitrogen in all commercial aircraft tires to eliminate the potential for water vapor from freezing at high altitudes. At altitude, air temps at the tires reach -30F. Any moisture in the tire can freeze, and if there is enough moisture in the tire, it can form ice and that can put the tire out of balance. Nitrogen doesn't form a liquid till -173C and pure nitrogen has almost no moisture.
Nitrogen does not expand or contract as much as oxygen, so the pressure inside the tyre does not alter as much between minus 50C at altitude and plus 200C or so when the tyre heats up as it hits the runway, compared to a tyre filled with air. Oxygen also reacts with rubber, and when this corrosion starts, the small particles break off and form rust and dust, causing them to leak. Nitrogen is far less reactive and it is not corrosive. Wheel surfaces stay smooth and clean, rubber remains supple and resilient.
Boeing has received reports of confirmed cases in which a wheel and tire assembly exploded when the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire. As a result, the U.S. Federal Aviation Administration issued Airworthiness Directive 87-08-09 requiring that only nitrogen be used to inflate airplane tires on braked wheels. However, tires may be topped off with air in remote locations where nitrogen may not be available if the oxygen content in the tire does not exceed 5 percent by volume.