The Doppler-limited absorption spectrum of 16O2^＋ cation was observed in the region of 11385-12100 cm^-1 by optical heterodyne velocity modulation absorption spectroscopy （OH-VMS）. The transitions were assigned to the （2, 19）, （3, 20）, and （5, 21） bands in the second negative system （A^2∏u-X^2∏g）. All the available lines measured using OH-VMS were global fitted in a nonlinear least-squares fitting procedure, and precise molecular constants （Bv, Av, Dv, pv, qv, γv） were obtained for the involved levels.
