The liquid caesium-oxygen system in the metallic regime has been investigated by the NMR technique. Measurements were made of the Knight shift for both ¹³³Cs and ¹⁷O nuclei, and the nuclear relaxation time, T₁, for the latter. The Knight shift for ¹³³Cs nuclei decreases almost linearly up to 25 at.% of O and further decreases with a steeper slope to the value in Cs₂O, while the shift for ¹⁷O nuclei is almost independent of the composition up to 21 at.%. The analysis of T₁ suggests that the gradient of the electrostatic field coupled with the nuclear quadrupole moment of ¹⁷O is also insensitive to the composition. The results are discussed with particular reference to the state of the O atoms and possible ionic association in the liquid Cs-O system in the metallic regime.