There are some reasons because oxygen gas is more reactive than nitrogen.
1. In the O₂ molecule there is a double bond between the two atoms of O. In the N₂ molecule, there is a triple bond between the two atoms of N. So it is easier to break a molecule of O₂ than to break the molecule of N₂.
2. Oxygen is more electronegative than nitrogen.
3. Oxygen needs 2 more electrons to fill its outer shell while nitrogen needs 3 more electrons to become stable. It is much easier for oxygen to complete the octet and to become stable, as compared to nitrogen.