The OPERA neutrino experiment in the underground Gran Sasso Laboratory (LNGS) was designed to perform the first detection of neutrino oscillations in direct appearance mode in the vμ→ v τ channel, the v τ signature being the identification of the τ -lepton created in its charged current interaction. The hybrid apparatus consists of a large mass emulsion film/lead target complemented by electronic detectors. Placed in the LNGS, it is exposed to the high-energy long-baseline CERN Neutrino beam to Gran Sasso (CNGS) 730 km away from the neutrino source. The observation of a first v τ candidate event was reported in 2010. In this paper, we discuss the result of the analysis of the data taken during the first two years of operation (2008-2009) underlining the major improvements brought to the analysis chain and to the Monte Carlo simulations. The statistical significance of the one event observed so far is then evaluated to 95%.