The NOvA experiment has made a $4.4sigma$-significant observation of $bar u_{e}$ appearance in a 2 GeV $bar u_{mu}$ beam at a distance of 810 km. Using $12.33times10^{20}$ protons on target delivered to the Fermilab NuMI neutrino beamline, the experiment recorded 27 $bar u_{mu} rightarrow bar u_{e}$ candidates with a background of 10.3 and 102 $bar u_{mu} rightarrow bar u_{mu}$ candidates. This new antineutrino data is combined with neutrino data to measure the oscillation parameters $|Delta m^2_{32}| = 2.48^{+0.11}_{-0.06}times10^{-3}$ eV$^2/c^4$, $sin^2 theta_{23} = 0.56^{+0.04}_{-0.03}$ in the normal neutrino mass hierarchy and upper octant and excludes most values near $delta_{rm CP}=pi/2$ for the inverted mass hierarchy by more than 3$sigma$. The data favor the normal neutrino mass hierarchy by 1.9$sigma$ and $theta_{23}$ values in the upper octant by 1.6$sigma$.
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