A revised absolute magnetic shielding scale for oxygen is established based on a recently reported highly precise experimental measurement of the O-17 spin-rotation constant in carbon monoxide. The isotropic oxygen magnetic shielding constant for (CO)-C-12-O-17 at the equilibrium geometry, sigma(e), is found to be -56.8+/-0.6 ppm. The experimental rovibrationally averaged value of the shielding constant at 300 K, (300 K), is -62.7+/-0.6 ppm, in excellent agreement with the ab initio value reported by Vaara [J. Chem. Phys. 109, 8388 (1998)]. Based on the revised scale and on experimentally known oxygen chemical shifts, (300 K) (H2O(l)) is 287.5+/-0.6 ppm and (300 K) (H2O(g)) is 323.6+/-0.6 ppm.