High spectral purity microwave oscillators are widely exploited in science and military areas including communication, radar, and navigation. Here, we theoretically analyze and experimentally observe the application of tunable electromagnetically induced transparency (EIT) effect generated within a single quasi-cylindrical microresonator (QCMR) to a high spectral purity microwave optoelectronic oscillator (OEO). Stable single-frequency microwave oscillation with phase noise of -123 dBc/Hz at 10 kHz offset from ~5 GHz carrier and -135 dBc/Hz at 100 kHz offset is demonstrated without using any narrow-band RF filters. Moreover, we evaluate the impact of laser-mode locking state, quality factor as well as spectral lineshapes of the EIT resonance, laser coupling efficiency, and three configurations of optical energy storage elements on the spectral purity of the oscillator, so as to improve its phase noise and stability performances. Extending the concept of EIT to a microwave generator opens a promising avenue towards compact low-phase-noise oscillator systems for emerging mass applications.