The OPERA experiment was designed to search for ν μ → ν τ oscillations in appearance mode, i.e., by detecting the τ leptons produced in charged current ν τ interactions. The experiment took data from 2008 to 2012 in the CERN Neutrinos to Gran Sasso beam. The observation of the ν μ → ν τ appearance, achieved with four candidate events in a subsample of the data, was previously reported. In this Letter, a fifth ν τ candidate event, found in an enlarged data sample, is described. Together with a further reduction of the expected background, the candidate events detected so far allow us to assess the discovery of ν μ → ν τ oscillations in appearance mode with a significance larger than 5σ .