We compare the time evolutions of two quantum systems: a quantum harmonic o
scillator driven by a classical force and two coupled quantum harmonic osci
llators. We find the conditions under which the first system can be conside
red as a 'mean-field approximation' to the second one. This happens in the
weak coupling limit provided that the 'master' oscillator was initially in
a highly excited quantum state with small fluctuations of the canonical var
iables (such as the coherent state or displaced thermal state), while the '
subordinate' oscillator must not be 'too excited'. The essential difference
between the resonance and nonresonance cases is discussed. In particular,
the effect of the resonance-quantum-state exchange is demonstrated.