  In the near future, extremely-large ground-based telescopes may conduct some
of the first searches for life beyond the solar system. High-spectral
resolution observations of reflected light from nearby exoplanetary atmospheres
could be used to search for the biosignature oxygen. However, while Earth's
abundant O$_2$is photosynthetic, early ocean loss may also produce high
atmospheric O$_2$ via water vapor photolysis and subsequent hydrogen escape. To
explore how to use spectra to discriminate between these two oxygen sources, we
generate high-resolution line-by-line synthetic spectra of both a habitable
Earth-like, and post-ocean-loss Proxima Centauri b. We examine the strength and
profile of four bands of O$_2$ from 0.63 to 1.27 $\mu$m, and quantify their
relative detectability. We find that 10 bar O$_2$ post-ocean-loss atmospheres
have 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 massive
O$_2$atmospheres, which is not present for the smaller amounts of oxygen
generated by photosynthesis. Consequently, any detection of the 1.27$\mu$m band
in reflected light indicates lower Earth-like O$_2$ levels, which suggests a
likely photosynthetic origin. However, the 0.69 $\mu$m O$_2$ band is relatively
unaffected 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 strategy
for observing and interpreting O$_2$ in exoplanet atmospheres, that could be
considered by future ground-based telescopes.
