  In the framework of the Lindblad theory for open quantum systems we determine
the degree of quantum decoherence and classical correlations of a harmonic
oscillator interacting with a thermal bath. The transition from quantum to
classical behaviour of the considered system is analyzed and it is shown that
the classicality takes place during a finite interval of time. We calculate
also the decoherence time and show that it has the same scale as the time after
which statistical fluctuations become comparable with quantum fluctuations.
