This paper presents the effect of oxygen on the microstructure of Co - 18 at. % Pt perpendicular medium. Oxygen addition to the CoPt films during high Ar pressure (4 Pa) sputtering successfully increased the perpendicular coercivity. 188 kA/m coercivity and 1.2 tesla (T) saturation magnetization were obtained. The perpendicular anisotropy energy was at least 570 kJ/m3. AEM and EDX measurements revealed that the film was composed of crystal grains and amorphous boundaries and that oxygen segregated clearly inside the amorphous boundaries. A TEM image showed a 10 nm grain diameter independent of the oxygen partial pressure. For CoPt films deposited under a lower Ar pressure (2 Pa), oxygen has been found to exist uniformly within the grains and boundaries and showed no significant effect on coercivity.