Absorption spectroscopy has been applied to the measurement of the arc-heater plumes. Number density distributions of atomic metastable oxygen were obtained from the measured absorption line profile at 777.19 nm in argon-oxygen plumes generated by arcjet type arc-heaters. As a result, it was found that the oxygen is localized off axis at the nozzle exit, and diffuses from outside toward the centerline in the plume. Moreover, numerical simulation was conducted to investigate the diffusion process of the oxygen in the arc-heater. The result indicates that the maximum degree of oxygen dissociation would be at the level of 0.01%.