KM3NeT/ORCA is a water Cherenkov detector under construction in the Mediterranean Sea, whose geometry has been optimised for the detection of [1, 100] GeV atmospheric neutrinos. After completion the ORCA detector will comprise an array of optical modules supported by 115 vertical Detection Units. Its main goals are the determination of the neutrino mass ordering and the measurement of the atmospheric neutrino oscillations. In addition ORCA is sensitive to tau neutrino appearance resulting from atmospheric neutrino oscillations. The full detector will collect a few thousands charged current tau neutrino events per year. In this contribution a first measurement is presented with ORCA6, an early subarray corresponding to about 5$\%$ of the final detector. A sample of 5828 neutrino candidates has been selected from the analysed exposure of 433 kton-years. With this dataset, the $\nu_\tau$~normalisation was measured to be $S_\tau = 0.48^{+0.5}_{-0.33}$, which translates into a \nutau~charged current cross section measurement of $\sigma_{\tau}^{\text{meas}}(E_{\nu}) = (2.5 ^{+2.6}_{-1.8}) \times 10^{-38} $cm$^{2}$ nucleon$^{-1}$ at the median charged current $\nu_\tau$ energy of 20.3~GeV. In addition, the unitarity of the neutrino mixing matrix was probed and allowed to improve the current limit on the non-unitarity parameter affecting the $\tau$-row of the mixing matrix, with $\alpha_{33}>$ -0.05 at the 2$\sigma$ confidence level. A larger data sample, consisting of 715 kton-years, yielded a measurement of the oscillation parameters as $\sin^2 \theta_{23} = 0.50 \pm 0.07$ and $\Delta m^2_{31}=-2.09^{+0.17}_{-0.21} \times 10^{-3} \, \text{eV}^2$.