I assume it is a L-C oscillator, not crystal-controlled.
There are many possible
explanations for this.
Insufficient buffering between the oscillator circuit
and the amplifier circuit.
Feedback from amplifier (need better shielding, power
supply decoupling, etc.)
Power supply regulation (if you have varactors or other
voltage dependant capacitances in our L-C) changes with addition of amplifier
If you post the circuit here on the forum, you will probably get more suggestions.