This paper reports the results of the experimental study of parameters for a DC oxygen discharge with water cathode in the pressure range of 0.1-1 bar and the discharge current of 40 mA. The radius of positive column, the cathode voltage drop, the cathode current density and the electric field strength were measured. Rotational temperatures of N-2 (C-3 Pi(u), V = 0) and OH (A(2)Sigma, V = 0) and absolute line intensities of atomic oxygen with wave length of 845 and 777 nm were determined as well. Plasma composition modeling was carried out by the combined solution of the Boltzmann equation for electrons, the equations of vibrational kinetics for ground states of N-2, O-2, H2O molecules, and the equations of chemical kinetics, and the plasma conductivity equation. Calculations were carried out taking into consideration the discharge radial heterogeneity and using experimental values of E/N and gas temperatures. The main particles being formed in plasma were shown to be (OH)-O-center dot, H2O2, O(P-3), O-2(a(1)Delta(g)), O-2(b(1)Sigma(+)(g)), H(S-1). On the basis of this calculation and experimental values of line intensities, the populating mechanism of (3p(3)P) level of atomic oxygen was discussed. The comparison of some properties of discharges in O-2, N-2 and air was done.