Oscillation controlled magnetic sensing by using magneto-resistance devices is proposed. An InSb magnetic sensing device was used in the feedback loop of a Hartley oscillator. The oscillation starts when the resistance of the InSb magnetic sensing device changes in the external magnetic field as the oscillation condition is satisfied. Output response and error rate in our magnetic sensing for the detection of oscillation were investigated. As a result a high output response and low noise were obtained. The resistance change dependence of the output voltage is in good agreement with calculation results obtained from F-matrix method. Noise originated from the output in the sensing circuit can be explained from the thermal noise and shot noise of total output impedance. Based on these analyses, S/N ratio of the oscillation controlled magnetic sensing is found to be higher than those of conventional methods. Furthermore, error rate less than 10-7 is found to be obtained, and it can be explained from calculated results of normal distribution taking the crest value of total noise, summarized thermal noise and shot noise, and threshold voltage into account. Therefore, this magnetic sensing method has potential application in highly sensitive magnetic sensors.