This paper presents an overview of the design and actual performance of an oxygen spectrometer for atmospheric science on a balloon, the OSAS-B.  This balloon-borne 4.7 THz spectrometer is used to measure atomic oxygen in the mesosphere and lower thermosphere, from 50 km to 500 km above the earth.  The spectrometer design addressed challenging weight and power limitations for operation down to -80° C.  The instrument incorporates sophisticated components including a superconducting hot-electron bolometer (HEB), a quantum cascade laser (QCL), and a digital fast Fourier transform spectrometer (DFFTTS) with 183 kHz resolution and 1.5 GHz bandwidth.  The instrument has 48 hour cryogen hold time, weighs 65 kg and consumes 90W in full operation.