In an earthquake the sudden release of energy due to the breaking and movement of the tectonic plate leads to ground shaking. The behavior and performance of buildings during an earthquake depends primarily on its' structural configuration. Nowadays, most buildings have irregularities both in-plan and in-elevation due to architectural needs and service requirements, which develops weak points in the structural system. Factors that influence the structure's seismic behavior include structural configuration, architectural layout, structure zonation and soil type. This study addresses the structural design and performance of buildings under seismic loads considering various types and magnitude of irregularities in-plan and in elevations as defined by the Oman Seismic Code. A ten-story high regular reinforced concrete building with a total height of 30 m, 20 m x 16 m plan dimension, 5 bays in the x-direction and 4 bays in the y-direction, located in muscat, Zone-1, and soil class-C. for irregularities in plan, six cases with torsional irregularity, three cases of floor discontinuity, and four cases of projection in-plan are used. Three cases of each weak story and soft story are considered for in-elevation irregularities. The study, therefore, discussed 19 different structural configurations in addition to the reference layout. All the cases are analyzed and designed under the seismic loads with the same conditions for gravity loads using ETABS. The building capacity and performance is estimated using pushover analysis, plastic hinges deformation, and capacity-demand curve. It is noticed that the seismic irregularities in the structure have a significant effect on structural performance, particularly the base shear, performance point, and plastic hinges formation compared to the regular structure. The torsional behavior for the structure was severe when a single shear wall is positioned at the periphery of the building with a 1.85 irregularity factor, compared to the other shear walls configurations and locations. For floor discontinuity, it is found that the centralized location of the floor opening is affecting the structural behavior generating a more vulnerable structure due to the reduction of base shear by 74% in the x-direction and 68% in the y-direction. The opening located in the center caused higher base shear over the opening at the periphery. With projection in plan, the performance of the building under seismic load will be affected due to the stress concentration in the projection direction. For irregularities in-elevation, soft story irregular models illustrate that increasing the first story height decreases the base shear value due to the stiffness difference between the first story and the storys above. Due to the non-continuity of vertical structural members at the first story, the structure's performance is affected by the reduction of building capacity goes into the collapse prevention stage. Irregularities will lead to a reduction in strength and stiffness in comparison to regular structures.