In order to investigate the enhancement mechanism of atmospheric-pressure oxygen pulsed discharge in a parallel-plate dielectric barrier discharge (DBD) with microstructures fabricated on the dielectric surface of the high-voltage electrode, this work systematically analyzes the electron transport processes, the formation and evolution of electric fields, and the spatial distribution of particles by using a two-dimensional fluid model. The introduction of microstructures can cause significant electric field distortion, generating a strong transverse electric field that locally confines and focuses electrons beneath the micro-structured region, leading to the formation of a stable corona-mode discharge. At the same time, the reduced local discharge gap near the microstructure enhances the longitudinal electric field, resulting in a temporal asynchrony between the corona discharge under the microstructure and the parallel-plate discharge in the adjacent flat regions. As the geometric dimensions of the microstructures increase, a secondary discharge is triggered, further modulating the overall discharge behavior. Under conditions where the corona discharge is suppressed by higher protrusions, the occurrence of secondary discharge effectively increases the proportion of high-energy electrons and the spatially averaged density of reactive oxygen atoms. Simulation results reveal that the corona discharge and the secondary discharge significantly raise electron density, electron temperature, and the proportion of high-energy electrons, thereby intensifying the discharge activity. These findings offer deep insight into the micro-mechanisms of microstructure-induced discharge enhancement and provide valuable guidance for designing highly efficient plasma devices with tailored geometric features.