Physical Sciences Inc. (PSI) proposes to design, build, test, and deliver to NASA a THz wavelength absorption sensor for continuous monitoring of atomic oxygen concentration in hypersonic flowfields. In a successful Phase I effort, PSI developed a THz wavelength Quantum Cascade Laser (QCL) at 63.2 microns, corresponding to a strong fine-structure transition of atomic oxygen. Using an external cavity design, we showed that the laser wavelength could be coarsely tuned to the atomic oxygen transition. Rapid and repeatable injection current tuning at this wavelength was also demonstrated. In the proposed Phase II program, the external cavity QCL design will be refined to include a wider continuous tuning range, higher laser operating temperature, and improved output power. The laser operation will be automated and integrated into a computer-controlled atomic oxygen sensor, providing continuous, real-time measurements of atomic oxygen concentration with a sensitivity of 10^13 atoms per cubic centimeter in a 10 Hz bandwidth. PSI will deliver, install, and test the sensor at the NASA Ames Aerodynamic Heating Facility, an arc-jet heated high-enthalpy flow facility.