KM3NeT/ORCA is a mega-ton volume neutrino detector and has entered the construction phase in the Mediterranean Sea. Optimised to study neutrino oscillations in the few GeV energy range, it has unprecedented sensitivity to oscillations of atmospheric muon and electron neutrinos into the tau neutrino channel. Tau neutrinos predominantly produce shower-like signatures, which can be identified on a statistical basis as an energy- and direction-dependent excess over the non-appearance hypothesis.

The contribution shows that the KM3NeT/ORCA detector will be able to exclude non-appearance already in the first months of data taking. In the longer run precise measurement of the tau normalisation will allow to put constraints on the commonly assumed unitarity of three neutrino mixing.