As a natural extension of the static Hall conductivity to ac regimes, the optical Hall conductivity σ_xy(ω) is studied theoretically in the THz region. We found that the Hall plateaus remain in the optical (THz) region when the disorder is not too large, although the quantization of the plateau height does not hold. We have also calculated σ_xy(ω) in the graphene QHE system to find that the Hall plateaus also remain with σ_xy(ω) reflecting the massless Dirac dispersion in graphene. We predict that the optical Hall conductivity should be measurable through an accurate detection of the Hall angle in the THz regime.