A plate-like circular monolayer molybdenum disulfide (MoS2) nanomechanical oscillator system based on Si/SiO2 substrate is proposed. An all-optical method, which uses a strong pump laser and a weak detection laser effect on the oscillator system at the same time, is proposed to measure mechanical oscillator frequency. The phenomenon of phonon-induced transparency is demonstrated in the system, and the physical interpretation is presented. By measuring the width between two peaks in the probe absorption spectrum, we find that the exciton-oscillator coupling strength is proportional to the width and the method can be used to measure the exciton-oscillator coupling strength. Further, an all-optical mass sensing scheme is proposed based on the nanomechanical oscillator system. By measuring the resonance frequency shift in optical spectrum, we can obtain the additional mass deposited on the surface of molybdenum disulfide oscillator directly. The simulation results show that the mass responsivity of the oscillator system is 2.32 Hz/ag. The monolayer molybdenum disulfide nanomechanical oscillator system may have potential applications in quantum sensing and all-optical MoS2-based devices.