Energy level diagram for atomic oxygen [O I]. In analysis of the oxygen line ratio, the 2972 and 2958 Å lines can be ignored as they only comprise 10% of transitions from the ¹S level to the ¹D level. Note that all atoms that decay through the 5577 Å line will then also decay through either the 6300 or 6364 Å line. Figure adapted from A. Bhardwaj & S. Raghuram (2012).