OPERA is a long-baseline neutrino experiment located in the Hall C of the underground Gran Sasso Laboratory at an average depth of 3.8 km.w.e., corresponding to muon energies at surface higher than 1.5 TeV. After a description of the main scientific goal of the experiment, we focus on the potentialities of OPERA used as a cosmic ray detector. In particular, we report on the measurement of the atmospheric muon charge ratio, on the analysis of upgoing muons induced by atmospheric neutrinos and on the recent coincidence study between the OPERA and LVD detectors.