We address the use of optical parametric oscillator (OPO) to counteract phase diffusion, and demonstrate phase-noise reduction for coherent signals traveling through a suitably tuned OPO. In particular, we theoretically and experimentally show that there is a threshold value on the phase noise, above which OPO can be exploited to "squeeze" phase noise. The threshold depends on the energy of the input coherent state, and on the relevant parameters of the OPO, i.e., gain and input-output and crystal loss rates.