We present the design of an organic solar cell (OSC), integrated with nanohole patterned aluminum electrode with hexagonal lattice and analyze the light absorption mechanism in detail. The periodic nanohole pattern in the electrode excites a resonant waveguide mode which localizes the electromagnetic fields into the active layer at the long wavelengths where the active material usually has a relatively low absorption coefficient. The calculated maximum achievable photocurrent density indicates that OSC integrated with nanohole patterned aluminum electrode shows slightly better performance compare to that of the conventional OSC integrated with an indium tin oxide electrode. Moreover, the waveguide mode can be tuned by varying the period of the nanohole array and the thickness of nanocrystalline zinc oxide for different kinds of cell structures. By combining with other strategies to enhance the absorption at the short wavelength, our finding provides a promising way to further improve the efficiency of the OSCs.