  We present an analysis of the process $\psi(3686) \to \Omega^-
\bar{\Omega}^+$ ($\Omega^-\to K^-\Lambda$, $\bar{\Omega}^+\to
K^+\bar{\Lambda}$, $\Lambda\to p\pi^-$, $\bar{\Lambda}\to \bar{p}\pi^+$) based
on a data set of $448\times 10^6$ $\psi(3686)$ decays collected with the BESIII
detector at the BEPCII electron-positron collider. The helicity amplitudes for
the process $\psi(3686) \to \Omega^- \bar{\Omega}^+$ and the decay parameters
of the subsequent decay $\Omega^-\to K^-\Lambda$ $(\bar{\Omega}^+\to
K^+\bar{\Lambda})$ are measured for the first time by a fit to the angular
distribution of the complete decay chain. The branching fraction of $\psi(3686)
\to \Omega^- \bar{\Omega}^+$ is measured to be $(5.82\pm 0.12\pm 0.24)\times
10^{-5}$, with an improved precision compared to previous measurements.
