Page "Electronic oscillator" Paragraph 11

The most common form of linear oscillator is an electronic amplifier such as a transistor or op amp connected in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide positive feedback.

When the power supply to the amplifier is first switched on, electronic noise in the circuit provides a signal to get oscillations started.

The noise travels around the loop and is amplified and filtered until very quickly it becomes a sine wave at a single frequency.