Oxygen occurs in the same proportion (a livable one, at that) in our air, all the time, unless the environmental conditions are drastically differing from what we normally consider 'normal'. Your home to outside, in a normal situation, would have no variance. However, it's probably safe to assume that the Oxygen levels would be lower on the side of a highway during rush-hour, while being in the middle of a forest would yield higher concentrations of Oxygen. Though how a human is effected by these differences would depend on how long they were exposed.