What was the oxygen concentration in the air during the late miocene period?
R. Tappert et al (2013) used carbon isotopes in ambers as proxies for atmospheric oxygen concentrations. Their work indicates an initial Miocene O2 concentration of 16% and rising to about 20% at the end:

It's pretty remarkable how different their results look from other models. Some think that some inferred upward trend in O2 levels since the Eocene gave rise to the mammals, or at least bigger brains in mammals.
Hope this helps!