  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
cm\textsuperscript{-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.
