The OPERA experiment was designed to study $nu_mutonu_tau$ oscillations in appearance mode in the CNGS neutrino beam. In this letter we report the final analysis of the full data sample collected between 2008 and 2012, corresponding to $17.97cdot 10^{19}$ protons on target. Selection criteria looser than in previous analyses have produced ten $nu_tau$ candidate events, thus reducing the statistical uncertainty in the measurement of the oscillation parameters and of $nu_tau$ properties. A multivariate approach for event identification has been applied to the candidate events and the discovery of $nu_tau$ appearance is confirmed with an improved significance level of 6.1 $sigma$. $Delta m^2_{23}$ has been measured, in appearance mode, with an accuracy of 20%. The measurement of $nu_tau$ CC cross-section, for the first time with a negligible contamination from $bar{nu}_tau$, and the first direct observation of the $nu_tau$ lepton number are also reported.
