A Message Passing Interface (MPI) parallel implementation of a hybrid solver that combines the Method of Moments (MoM) with higher order basis functions and Physical Optics (PO) has been successfully used to solve a challenging problem including a 2160-slot waveguide array on an airplane with a maximum dimension larger than 1000 wavelengths. The block-partitioned scheme for the large dense MoM matrix combined with the process-cyclic scheme for the PO discretized triangles is designed to achieve excellent load balance and high parallel efficiency. To break the limitation of physical memory, the parallel out-of-core technique is introduced to tackle large dense systems generated using the MoM formulation. This research provides a solution with reasonable accuracy for solving large on-board antenna problems but has very low memory usage.