The question of phase measurement for a quantum oscillator using the q
uantum non-demolition principle (NDP), which does not need the use of
the wave packet reduction postulate, is studied. It is shown that this
approach is appropriate to introduce in a proper way the relevant obs
ervables for unlocalized (unsharp) measurement of quantum phase. Our a
nalysis suggests that in contrast to other quantum-mechanical problems
(e.g. the position and the momentum of a particle), the operator to r
epresent the phase of a quantum field cannot be separated from the mea
surement process. Thus, the quantum phase operator must be defined for
a corresponding measurement scheme. Moreover, it allows us to describ
e the time continuous process of quantum phase observations and also t
o study the stochastic dynamics of the oscillator under such measureme
nt.