Polycrystalline diamond thin films are grown on a p-type Si (100) single crystal substrate at a low surface deposition temperature of 455 degrees C using a microwave plasma enhanced chemical vapor deposition process in an Ar-rich Ar/H-2/CH4 plasma containing different oxygen levels from 0% to 0.75%. The surface deposition temperatures are measured and monitored by an IR thermometer capable of working in a plasma environment without any interference from the plasma emissions. The lower surface deposition temperature at high microwave power of 1300 W and higher gas pressure of 95 torr is achieved by active cooling of the substrate from the backside using a specially designed cooling stage. An enhanced growth rate from 0.19 to 0.63 mu m/h is observed with varying oxygen from 0% to 0.75% in the plasma. Diamond grain size also increased from 0.69 mu m for the sample with no oxygen to 1.74 mu m for the sample with 0.75% oxygen. The diamond films are characterized using x-ray diffraction, environmental scanning electron microscopy field emission gun, Raman spectroscopy, and x-ray photoelectron spectroscopy. The enhanced growth rate is correlated with the enhanced atomic hydrogen to C-2 ratio with increasing oxygen concentration in the plasma, which is measured by an in situ optical emission spectroscopy.