Oxygen concentration in the Si melt of a Czochralski system was measured by immersing an oxygen sensor in the melt. The measurement clarified the existence of an inhomogeneous distribution of oxygen whose pattern was similar to the temperature distribution in the melt. This result suggests that the oxygen profile in the Si melt is not axisymmetric, although the furnace structure is axisymmetric. This oxygen fluctuation is proposed to be one reason for striations appearing in the grown crystals.