Concept of the oscillator

For the circuit to oscillate the gain should be high, the feedback should be positive, and the coupling from output to input should be good. The feedback path should be easy for a signal to follow. The phase of a fed back signal is reversed without any difficulty, so that common emitter or common source amplifiers are made to oscillate.

Feedback at a single  frequency

Recalling public-address fiasco, some variation of which you have doubtless heard several times, could you know in advance whether feedback would have a low pitch, a midrange pitch? No. The oscillation was not intended, and may have started from any audio frequency.The frequency of the oscillator is controlled by means of resonant or tuned, circuits. These are usually inductance capacitance (LC) or resistance capacitance (RC) combinations. The LC scheme is common at RF; the RC method is more often used for audio oscillators.
The tuned circuit makes feedback path easy for the signal to follow at one frequency, but difficult to follow at all other frequencies. The result is that oscillation happen at a predictable and stable frequency can be determined by the inductance and capacitance or by resistance and capacitance.

                                                                 Figure-- Basic concept of the oscillator.