  The future extremely large telescopes (ELTs) are expected to be powerful
tools to probe the atmospheres of extrasolar planets using high-dispersion
spectroscopy, with the potential to detect molecular oxygen in Earth-like
planets transiting nearby, late-type stars. So far, simulations have
concentrated on the optical 7600 \AA{} A-band of oxygen using synthetic noise
distributions. In this paper, we build upon previous work to predict the
detectability of molecular oxygen in nearby, temperate planets by using
archival, time-series data of Proxima Centauri from the high-dispersion UVES
spectrograph on ESO's Very Large Telescope (VLT). The brightest transiting
M-dwarfs are expected to be about 25 times fainter than Proxima, a factor that
is similar to the difference in light-gathering power between the VLT and the
future ELTs. By injecting synthetic oxygen transmission signals into the UVES
data, the $\mathrm{O}_2$ detectability can be studied in the presence of real
data with real noise properties. Correcting for the relatively low throughput
($\sim$4%) of the Proxima spectra to an assumed 20% throughput for a
high-dispersion spectrograph on the European ELT, we find that the molecular
oxygen signature of an Earth-twin transiting a nearby ($d \approx 7
\,\mathrm{pc}$) M5V star can be detected in 20-50 transits (a total of 70-175
hours of observing time). This estimate using more realistic simulations is
close to previous predictions. Novel concepts that increase the instrumental
throughput can further reduce the time span over which such observations need
to be taken.
