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In the near future, extremely-large ground-based telescopes may conduct someof the first searches for life beyond the solar system. High-spectralresolution observations of reflected light from nearby exoplanetary atmospherescould be used to search for the biosignature oxygen. However, while Earth'sabundant O$_2$is photosynthetic, early ocean loss may also produce highatmospheric O$_2$ via water vapor photolysis and subsequent hydrogen escape. Toexplore how to use spectra to discriminate between these two oxygen sources, wegenerate high-resolution line-by-line synthetic spectra of both a habitableEarth-like, and post-ocean-loss Proxima Centauri b. We examine the strength andprofile of four bands of O$_2$ from 0.63 to 1.27 $\mu$m, and quantify theirrelative detectability. We find that 10 bar O$_2$ post-ocean-loss atmosphereshave strong suppression of oxygen bands, and especially the 1.27$\mu$m band.This suppression is due to additional strong, broad O$_2$-O$_2$collisionally-induced absorption (CIA) generated in these more massiveO$_2$atmospheres, which is not present for the smaller amounts of oxygengenerated by photosynthesis. Consequently, any detection of the 1.27$\mu$m bandin reflected light indicates lower Earth-like O$_2$ levels, which suggests alikely photosynthetic origin. However, the 0.69 $\mu$m O$_2$ band is relativelyunaffected by O$_2$-O$_2$ CIA, and the presence of an ocean-loss high-O$_2$atmosphere could be inferred via detection of a strong 0.69 $\mu$m O$_2$ band,and a weaker or undetected 1.27 $\mu$m band. These results provide a strategyfor observing and interpreting O$_2$ in exoplanet atmospheres, that could beconsidered by future ground-based telescopes.

High-Resolution Spectral Discriminants of Ocean Loss for M Dwarf Terrestrial Exoplanets