  The discrimination of the two possible options for the neutrino mass ordering
(normal or inverted) is a major goal for current and future neutrino
oscillation experiments. Such goal might be reached by observing
high-statistics energy-angle spectra of events induced by atmospheric neutrinos
and antineutrinos propagating in the Earth matter. Large volume water-Cherenkov
detectors envisaged to this purpose include the so-called KM3NeT-ORCA project
(in seawater) and the IceCube-PINGU project (in ice). Building upon a previous
work focused on PINGU, we study in detail the effects of various systematic
uncertainties on the ORCA sensitivity to the mass ordering, for the reference
configuration with 9 m vertical spacing. We point out the need to control
spectral shape uncertainties at the percent level, the effects of better priors
on the theta-23 mixing parameter, and the benefits of an improved flavor
identification in reconstructed ORCA events.
