Efficient O3 oxidation of NO is crucial for integrated removal of NOx and SO2 from marine diesel engines. This study focuses on a wet scrubber for a 140 kW engine, designing an in-scrubber device for O3 oxidation of NO. By controlling parameters such as O3 flow rate, inlet temperature, inlet diameter, and inlet angle, the characteristics of NO/NO2 distribution fields, temperature fields, and velocity fields within the reactor were systematically investigated. Results demonstrate that increasing the O3 flow rate or reducing the O3 inlet diameter effectively enhances NO oxidation efficiency and promotes uniform NO2 distribution within the device. A specific number of deflector plates optimizes both the NO2 distribution field and the internal velocity field. This study provides critical theoretical and practical guidance for integrated wet removal technologies targeting NOx and SO2 in marine diesel engines.