Three simple spectrophotometric methods have been described for the assay of olanzapine in its pure and pharmaceutical formulations. The direct method (A) is based on the drug oxidation with excess of N-bromosuccinimide in acidic medium and the two indirect methods (B and C) are based on the oxidation of the drug with excess of N-bromosuccinimide and cerium(IV)sulfate, followed by the reaction of the unconsumed oxidants with celestine blue. The calibration graphs were linear over the range 10 - 120 µg mL^-1 (method A), 0.5 - 6.0 µg mL^-1 (method B) and 0.6 - 3.0 µg mL^-1 (method C). After validation, the proposed methods were successfully applied to assay of olanzapine in its commercial tablets with mean percentage recoveries of 101.23 ± 0.10, 96 ± 0.10 and 94 ± 0.04%. The mechanism of olanzapine oxidation with N-bromosuccinimide was also proposed.