for an oscillator, feedback ratio should br positive as per barkhausen criteria...further there exists two cases...what will happen when the ratio is exactly equal to one...when it is greater than one...
There is allways some random noise in circuits such as thermal noise and RF signals so at gain=1 the noise energy will circulate and increase as there is more noise over time. The noise energy at will the frequency the circuit is designed to oscillate at will be amplified more than the nosie at other frequencys. The energy in the circuit will increase until it is stopped by some non-linear effect such as the supply voltage of the circuit.