[stevetron]

In some reading I did several (decades) years ago, I read how the invention of the oscillator was an accident. Someone was building an amplifier, and made a wiring error that was described as connecting the amplifier's input to the amplifier's output and it produced a squeal.

Up until that time, radio frequency carriers were produced with AC Generators designed for a high-frequency (> 60 Hz) output. That is in part why it took so long for things like music to be broadcast by radio: you needed a high-frequency carrier. Generators were topping out at about 15K Hz according to the old article I read.

So once they had the accidental circuit that squealed, it was studied and expermented upon (trial and etrror modifications), and that is where all the 'classic' oscillator circuits originate.

Essentially, an oscillator is an amplifier with positive feedback. Amplifiers that are amplifiers sometimes have negative feedback.