ORCA (Oscillations Research with Cosmics in the Abyss) is the low-energy branch of KM3NeT, the next generation underwater Cherenkov neutrino detector in the Mediterranean Sea. Its primary goal is to resolve the long-standing unsolved question of whether the neutrino mass ordering is normal or inverted by measuring matter oscillation effects with atmospheric neutrinos. The ORCA design foresees a dense configuration of KM3NeT detection units, optimised for studying the interactions of neutrinos in seawater at energies between few and 100~GeV. Multi-PMT optical modules will exploit the excellent optical properties of deep seawater to accurately reconstruct both cascade (mostly electron neutrinos) and track (mostly muon neutrinos) events. The potential of the ORCA detector both in neutrino mass ordering studies and in obtaining new constraints on other key parameters such as $\theta_{23}$ will be discussed.