what is the oscillator circuit?
what caused the transient?
what are the characteristics of the transient?
without any information, i'll go out in a limb:
oscillators are designed to produce a single frequency. any oscillation outside that frequency dies off really fast (in a well designed oscillator)
if the frequency is adjustable, the transition from frequency to frequency will happen really fast (in a well designed oscillator)
really fast means faster than the user can see, so the user can see that the change occurred, but not see any intermediate state.
sometimes the transition can take a while. this is okay, if it isn't a problem for the following circuits or function