Abstract A tunable optical oscillator that generates signals at the micro- to millimeter-wave band for wireless communication applications is suggested. It uses directly modulated semiconductor lasers, in which sideband modes and four-wave mixing (FWM) conjugate modes are injection locked by the simple control of the applied modulation power. The signals at 15 GHz with phase noise of below −95 dBc/Hz at an offset frequency of 100 kHz were experimentally obtained. The frequency of the generated signal is tunable, and the maximum achievable signal frequency is limited mainly by the bandwidth of the receiver.