The output amplitude is determined by the gain of the amplifier and the feedback circuit.

Oscillators can produce sine waves, the frequency of which is determined by TUNED CIRCUITS.
Tuned circuits consist of a capacitor and inductance.

Square wave oscillators use resistors and capacitors to determine the frequency of oscillation.

Ideally the frequency of an oscillator should be stable, but due to temperature variations and mechanical vibration this may not be so.

Precautions are taken against frequency DRIFT.

"Howl round", caused by placing a microphone too close to a loudspeaker, is an audio oscillation caused by positive feedback.