[Roju (OP)]

Recently I moved to the north of Norway, so I've seen quite a few auroras. Because the colour is mainly green, I was sure this must be due to electron transitions in N2 (because the atmosphere is like 75% N2)
Looking into this, however, I found out that the colours of the aurora are mainly due to electron transitions in atomic oxygen gas. Why is it that the electron transitions of oxygen are most prevalent in the colours of the aurora, even though it is far less abundant than nitrogen?