The KM3NeT Collaboration is constructing two water-Cherenkov neutrino detectors at the bottom of the Mediterranean Sea: ARCA, which is designed for neutrino astronomy in the TeV to PeV range, and ORCA, optimised for GeV neutrino detection. The ORCA detector will comprise 115 string-like vertical Detection Units arranged in a cylindrical array. Its main objectives are to determine the neutrino mass ordering and measuring atmospheric neutrino oscillations, and to probe physics beyond the Standard Model. During 2020 and 2021, an early configuration of the detector with six lines was in operation. A high-purity neutrino sample covering 433 kton-years of exposure was extracted using optimised reconstruction algorithms and machine learning classifiers. In this seminar, the basics of the phenomena studied, how the ORCA detector works, the treatment of the data and the details of the measurements will be described. In particular, the measurements of the neutrino oscillation parameters (the atmispheric mixing angle and mass splitting), as well as the invisible neutrino decay rate constant will be reported.