  The Omega- has an extremely long lifetime, and is the most stable of the
baryons with spin 3/2. Therefore the Omega- magnetic moment is very accurately
known. Nevertheless, its electric quadrupole moment was never measured,
although estimates exist in different formalisms. In principle, lattice QCD
simulations provide at present the most appropriate way to estimate the Omega-
form factors, as function of the square of the transferred four-momentum, Q2,
since it describes baryon systems at the physical mass for the strange quark.
However, lattice QCD form factors, and in particular GE2, are determined at
finite Q2 only, and the extraction of the electric quadrupole moment, Q_Omega=
GE2(0) e/(2 M_Omega), involves an extrapolation of the numerical lattice
results. In this work we reproduce the lattice QCD data with a covariant
spectator quark model for Omega- which includes a mixture of S and two D states
for the relative quark-diquark motion. Once the model is calibrated, it is used
to determine Q_Omega. Our prediction is Q_Omega= (0.96 +/- 0.02)*10^(-2) efm2
[GE2(0)=0.680 +/- 0.012].
