This study presents a parametric investigation of how localized defects specifically circular delamination and geometric imperfections affect the buckling and post-buckling performance of composite stiffened panels. Delamination propagation is simulated in Abaqus/CAE using finite element analysis (FEA) in the panels under cohesive zone modeling. By means of both linear buckling and nonlinear post-buckling simulations, the impact of important parameters such as delamination size, position, and imperfection amplitude is investigated. Results show that combined defects significantly reduce the buckling load, particularly when delamination occurs beneath the stiffener. Larger defects trigger an unstable post-buckling response, with cohesive modeling capturing progressive damage accumulation. Especially in reproducing trends in load degradation and crack development, comparisons with experimental data from literature confirm the predictive accuracy of the simulations. These results underline the need of including reasonable defect conditions into load-bearing composite construction design evaluations. Recommendations are proposed to guide defect tolerance criteria and inform inspection protocols in aerospace applications.