The role of oxygen in fluorocarbon etching plasmas is investigated using sub millimeter wavelength absorption spectroscopy. The plasmas were created in a specially modified capacitively coupled Gaseous Electronics Conference (GEC) Reference Reactor with a commercial electrostatic chuck. Photoresist and SiO@sub 2@ blanket coated wafers were etched in C@sub 4@F@sub 8@/O@sub 2@/Ar, C@sub 5@F@sub 8@/O@sub 2@/Ar, and C@sub 4@F@sub 6@/O@sub 2@/Ar discharges. The absolute density of various radicals (CF, CF@sub 2@, CHF@sub 3@, COF@sub 2@, CO, etc.) were measured as a function of the percentage of oxygen in the feed gas mixture using a sub millimeter source based on a 48x frequency multiplication chain. These results are also compared to the oxygen containing fluorocarbon gas C@sub 4@F@sub 8@O.