A pure oxygen certified reference material, NMIJ CRM 3404-c, was developed by National Metrology Institute of Japan, mainly for oxygen standard gases supplied by Japan Calibration Service System. The certified value for oxygen in the reference material was obtained by a paramagnetic oxygen analyzer, which was calibrated by standard gases prepared using a dynamic volumetric method with mass-flow controllers. The standard gases were prepared by mixing volumetrically pure nitrogen with NMIJ CRM 3404-a, whose certified value was obtained by a subtraction method. The uncertainty of the certified value of NMIJ CRM 3404-c originated from the repeatability of the oxygen analyzer. Standard deviation of the repeatability is 2.48 mV, which corresponds to 1.2 μmol mol⁻¹. An example of the certified value of NMIJ CRM 3404-c is (1.0000000±3.1×10⁻⁶) mol mol⁻¹ (k = 2).