Frequencies for 32 P-branch transitions of the O 2 A-band (b1Σg+←X3Σu-) were measured between 12975 and 13115cm −1 using the frequency-stabilized cavity ring-down spectrometer located at National Institute of Standards and Technology, Gaithersburg, MD. Absolute line positions were calibrated using the hyperfine components of 39 K D 1 and D 2 atomic transitions yielding uncertainties <1MHz (∼3×10 −5 cm −1 ). Spectroscopic parameters for the b1Σg+(v=0) state were calculated and compared with previously determined values. These new transition frequencies should serve as a convenient secondary calibration standard in the 760nm region.