Hey there! :)
If you've ever worked with coefficient's in math, you would determine it as a number in front of a variable.
For example, in 4x the coefficient would be 4.
So, in O2, it would be 1, because there is no given value. When there is no given value- you can always assume that the number is 1. 1*02= still equals O2.
I hope this helps, DM me with any questions if you have them. :)
~kaikers
