Abstract The CW-Cavity Ring Down Spectroscopy (CW-CRDS) technique has been used to record the high sensitivity absorption spectrum of the a 1Δ g– X 3 Σ g − band of oxygen near 1.27 μm. The spectra were obtained between 7640 and 7917 cm −1 with “natural” oxygen and with a sample highly enriched in 18O. The absolute intensities of 376 and 643 oxygen transitions were measured in the two spectra. They include the a 1Δ g− X 3 Σ g − (0–0) bands of 16O 2, 16O 18O, 16O 17O, 17O 18O and 18O 2. The (0–0) bands of 16O 2 and 18O 2 show weak quadrupole transitions with line intensities ranging from 1×10 −30 to 1.9×10 −28 cm/molecule. They are accompanied by the a 1Δ g− X 3 Σ g − (1–1) hot bands, which are reported for the first time. The line profiles of the transitions of the 16O 17O and 17O 18O isotopologues were observed to be broadened due to an unresolved magnetic hyperfine structure. Accurate spectroscopic parameters of the different energy levels involved in the observed bands were derived from a global fit of the observed line positions, combined with microwave and Raman measurements available in the literature.