In situ high P-T Raman measurements and optical observations of solid and fluid oxygen up to 1250 K between 8 and 25 GPa reveal the existence of a new molecular phase eta and strikingly unusual behavior of the melting curve. Three triple points were also identified in the P-T domain of the new phase. The data give a direct measure of the melting curve that greatly extends previous optical investigations. We find the melting temperature is significantly higher than that indicated by the existing phase diagram (e.g., 400 K higher at 25 GPa). Raman measurements in low and high frequency regions reveal the extent of orientational order disorder and persistence of strong intermolecular interactions in the high P-T phases.