The next generation of water Cherenkov neutrino telescopes in the Mediterranean Sea are under construction offshore France (KM3NeT/ORCA) and Sicily (KM3NeT/ARCA). The KM3NeT/ORCA detector features an energy detection threshold which allows to collect atmospheric neutrinos to study flavour oscillation. This paper reports the KM3NeT/ORCA sensitivity to this phenomenon. The event reconstruction, selection and classification are described. The sensitivity to determine the neutrino mass ordering was evaluated and found to be 4.4σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ if the true ordering is normal and 2.3σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ if inverted, after 3 years of data taking. The precision to measure σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ and σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ were also estimated and found to be σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ and σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ for normal neutrino mass ordering and, σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ and σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ for inverted ordering. Finally, a unitarity test of the leptonic mixing matrix by measuring the rate of tau neutrinos is described. Three years of data taking were found to be sufficient to exclude σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ event rate variations larger than 20% at σσΔm322θ2385.10-6eV2(-3.1+1.9)∘75.10-6eV2(-7.0+2.0)∘3σ level.