Your elemental numbers for Oxygen and Nitrogen atoms are bang on, but those two elements are bonded in pairs in normal chemical state of the gases. (apparently I am better at chemistry at 44 than I was at 16...LOL)
It seems that the bond between the two Oxygen atoms is just slightly more compact than the bond between the Nitrogen atoms.
Think of the rubber in the tire as a filter media... We are talking about portions of a billionth of Meter, Dioxygen molecules have a smaller crosssection compared to Dinitrogen therefore they slip through the rubber more readily.
The link below is what has helped me. Now it is from a supporter of nitrogen but I would think the physics and chemistry of basic elements could not be construed.
http://www.getnitrogen.org/pdf/graham.pdf