  Auroral emissions provide opportunities to study the tenuous atmospheres of
Solar System satellites, revealing the presence and abundance of molecular and
atomic species as well as their spatial and temporal variability. Far-UV
aurorae have been used for decades to study the atmospheres of the galilean
satellites. Here we present the first detection of Europa's visible-wavelength
atomic oxygen aurora at 6300/6364 \AA{} arising from the metastable O$(^1$D)
state, observed with the Keck I and Hubble Space Telescopes while Europa was in
eclipse by Jupiter on six occasions in February-April 2018. The disk-integrated
O($^1$D) brightness varies from $<$500 R up to more than 2 kR between dates, a
factor of 15 higher than the OI 1356 \AA{} brightness on average. The ratio of
emission at 6300/5577 \AA{} is diagnostic of parent molecule; the 5577 \AA{}
emission was not detected in our dataset, which favors O$_2$ as the dominant
atmospheric constituent and rules out an O/O$_2$ mixing ratio above 0.35. For
an O$_2$ atmosphere and typical plasma conditions at Europa's orbit, the
measured surface brightness range corresponds to column densities of
1-9$\times$10$^{14}$ cm$^{-2}$.
