In this paper the classical theory of Above Threshold Ionization (ATI) in the oxygen plasma was used to show how the residual electron energy depends on the laser parameters such as pulse length, wavelength and peak intensity. The value of ATI energy is found to increase with laser wavelength and its intensity. Our study conducted for three cases of τ > 2π/νp, τ = 2π/ωp, and τ < 2π/ωp, where ωp is the plasma frequency, reveals that the ATI energy is decreased for the pulse duration τ ≠ 2π/ωp. Also it is showed how the space charge effect can reduce the residual electron energy to a minimum value, in a suitable condition. By optimizing various parameters, we can generate the cold electrons suitable for the recombination x-ray laser.