The OPERA experiment, designed to perform the first observation of νμ→ντ oscillations in appearance mode through the detection of the τ leptons produced in ντ charged current interactions, has collected data from 2008 to 2012. In the present paper, the procedure developed to detect τ particle decays, occurring over distances of the order of 1 mm from the neutrino interaction point, is described in detail and applied to the search for charmed hadrons, showing similar decay topologies as the τ lepton. In the analysed sample, 50 charm decay candidate events are observed while 54±4 are expected, proving that the detector performance and the analysis chain applied to neutrino events are well reproduced by the OPERA simulation and thus validating the methods for ντ appearance detection.