We present a detailed visible and near-IR spectro-interferometric analysis of the Be-shell star $omicron$ Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations. We measured the stellar radius of $omicron$ Aquarii as 4.0 $pm$ 0.3 $mathrm{R_{odot}}$. We constrained the disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The disk sizes in H$alpha$ and Br$gamma$ were found to be similar, at $sim$10-12 $mathrm{D_{star}}$, which is uncommon since most results for Be stars show a larger extension in H$alpha$ than in Br$gamma$. We found that the inclination angle $i$ derived from H$alpha$ is significantly lower ($sim$15 deg) than the one derived from Br$gamma$. The disk kinematics were found to be near to the Keplerian rotation in Br$gamma$, but not in H$alpha$. After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the disk in both lines: $Sigma_{0}$ = 0.12 g cmtextsuperscript{-2} and $m$ = 3.0. The stellar rotational rate was found to be very close ($sim$96%) to the critical value. Our analysis of multi-epoch H$alpha$ profiles and imaging polarimetry indicates that the disk has been stable for at least 20 years. Compared to Br$gamma$, the data in H$alpha$ shows a substantially different picture that cannot fully be understood using the current physical models of Be star disks. $omicron$ Aquarii presents a stable disk, but the measured $m$ is lower than the standard value in the VDD model for steady-state. Such long-term stability can be understood in terms of the high rotational rate for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.
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