The small signal gain of the liquid oxygen around 1580 nm is measured using a tunable optical parametric oscillator (OPO) system. A small signal gain of 0.12 cm-1 is obtained at the wavelength of 1579.9 nm. The profile of small signal gain from 1579.2 nm to 1580.8 nm is also presented. The high positive gain indicates that the liquid oxygen is a potential medium for high energy laser. A comprehensive parameter optimization must be done in order to improve the small signal gain.