Polycrystalline diamond growth by microwave plasma chemical vapor deposition (MPCVD) at high-pressure (34.5 kPa) was investigated. The CH4/H2/O2 plasma was detected online by optical emission spectroscopy (OES), and the spatial distribution of radicals in the CH4/H2/O2 plasma was studied. Raman spectroscopy was employed to analyze the properties of the diamond films deposited in different oxygen volume fraction. The uniformity of diamond films quality was researched. The results indicate that the spectrum intensities of C2, CH and Hα decrease with the oxygen volume fraction increasing. While the intensity ratios of C2, CH to Hα also reduced as a function of increasing oxygen volume fraction. It is shown that the decrease of the absolute concentration of carbon radicals is attributed to the rise volume fraction of oxygen, while the relative concentration of carbon radicals to hydrogen atom is also reducing, which depressing the growth rate but improving the quality of diamond film. Furthermore, the OH radicals, role of etching, its intensities increase with the increase of oxygen volume fraction. Indicated that the improvement of OH concentration is also beneficial to reduce the content of amorphous carbon in diamond films. The spectrum space diagnosis results show that under high deposition pressure the distribution of the radicals in the CH4/H2/O2 plasma is inhomogeneous, especially, that of radical C2 gathered in the central region. And causing a rapid increase of non-diamond components in the central area, eventually enable the uneven distribution of diamond films quality.