The solid electrolyte interphase (SEI) on a Li metal anode is important in protecting the Li from further oxidation. Previous work has demonstrated that the SEI formed in a lithium-air battery (LAB) can be tailored depending on the gas species present. Oxygen helps create an oxide-rich SEI and promotes uniform Li metal deposition as opposed to dendritic Li which are formed when only argon is present. In order to compare the protective quality of the SEI formed under O₂ or under Ar, we used TEMPO, (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, as a probe. First, we studied the possible reduction products of TEMPO. We found that TEMPO initially reduces to TEMPO^-, then undergoes further reductive degradation such as ring opening or formation of amines. Next, we exposed a Li metal electrode with SEI formed either under O₂ or Ar to an electrolyte solution containing TEMPO. We characterized the ability of the SEI to prevent TEMPO reduction using operando Electron Paramagnetic Resonance (EPR) spectroscopy as well as ex situ solid-state and solution Nuclear Magnetic Resonance (NMR) spectroscopy. We show that the concentration of TEMPO in the electrolyte decreases more slowly and fewer TEMPO reduction products are formed when the Li metal has an oxide-rich SEI. This suggests that the oxide-rich SEI is better at protecting the Li metal from oxidation and that TEMPO is a suitable probe for SEI quality.