  Omicron Aquarii is late-type, Be shell star with a stable and nearly
symmetric H$\alpha$ emission line. We combine H$\alpha$ interferometric
observations obtained with the Navy Precision Optical Interferometer (NPOI)
covering 2007 through 2014 with H$\alpha$ spectroscopic observations over the
same period and a 2008 observation of the system's near-infrared spectral
energy distribution to constrain the properties of $o$~Aqr's circumstellar
disk. All observations are consistent with a circumstellar disk seen at an
inclination of $75\pm\,3^{\circ}$ with a position angle on the sky of
$110\pm\,8^{\circ}$ measured E from N. From the best-fit disk density model, we
find that 90\% of the H$\alpha$ emission arises from within $9.5$ stellar
radii, and the mass associated with this H$\alpha$ disk is $\sim
1.8\times10^{-10}$ of the stellar mass and the associated angular momentum,
assuming Keplerian rotation for the disk, is $\sim 1.6\times10^{-8}$ of the
total stellar angular momentum. The occurrence of a central quasi-emission
(CQE) feature in Mg\,{\sc ii} $\lambda\,4481$ is also predicted by this
best-fit disk model and the computed profile compares successfully with
observations from 1999. To obtain consistency between the H$\alpha$ line
profile modelling and the other constraints, it was necessary in the profile
fitting to weight the line core (emission peaks and central depression) more
heavily than the line wings, which were not well reproduced by our models. This
may reflect the limitation of assuming a single power-law for the disk's
equatorial density variation. The best-fit disk density model for $o$~Aqr
predicts that H$\alpha$ is near its maximum strength as a function of disk
density, and hence the H$\alpha$ equivalent width and line profile change only
weakly in response to large (factor of $\sim 5$) changes in the disk density.
