  We present high spectral resolution optical spectra obtained with the ARCES
instrument at Apache Point Observatory showing detection of the [OI]6300 A line
in interstellar comet 2I/Borisov. We employ the observed flux in this line to
derive an H$_2$O production rate of (6.3$\pm$1.5)$\times$10$^{26}$ mol
s$^{-1}$. Comparing to previously reported observations of CN, this implies a
CN/H$_2$O ratio of $\sim$0.3-0.6%. The lower end of this range is consistent
with the average value in comets, while the upper end is higher than the
average value for Solar System comets, but still within the range of observed
values. C$_2$/H$_2$O is depleted, with a value likely less than 0.1%. The
dust-to-gas ratio is consistent with the normal value for Solar System comets.
Using a simple sublimation model we estimate an H$_2$O active area of 1.7
km$^2$, which for current estimates for the size of Borisov suggests active
fractions between 1-150%, consistent with values measured in Solar System
comets. More detailed characterization of 2I/Borisov, including compositional
information and properties of the nucleus, is needed to fully interpret the
observed H$_2$O production rate.
