An optoelectronic oscillator (OEO) based on a double-Brillouin-frequency shifter (DBFS) is proposed and experimentally demonstrated. Two orders of stimulated Brillouin scattering are realized in the DBFS, and the narrow gain bandwidth of the second-order Stokes wave is utilized to implement the phase modulation to intensity modulation conversion. Only one laser is used to provide both signal and pump waves. A k-band microwave signal with a harmonic suppression ratio of 61 dB and a side-mode suppression ratio of 45 dB is experimentally generated, and 400-MHz tunability is realized by adjusting the laser wavelength. Because of the simple structure and high-frequency oscillation, the DBFS-based OEO can find applications for wireless communications and civil radar systems.