From a Phd dissertation here, I read about the material follows:
"Odd oxygen is a measure of oxidative capacity of the atmosphere. It contains species with an oxygen atom avaliable to serve as an oxidizing agent."
The author summed the species as $O_3$, $O$, $O^1D$, $NO_2$ and $2\ NO_3$, etc as the whole odd oxygen.
The species which has more than one reactive oxygen atoms are counted multiple times to represent he total number of reactive oxygen atoms.
How to analysis the amout of reactive oxygen atom of certain species like $NO_2$ and $2\ NO_3$