Co-flow water cavitating jets induce compressive residual stress through cavitation impacts produced by the collapse of the cavitation cloud. Co-flow water cavitation peening causes minimal surface alteration compared to conventional processes such as shot peening, which is a major advantage. However, enhancement of cavitation intensity for co-flow water cavitation peening nozzles is required for practical applications requiring greater process capability. Scaling of co-flow cavitation peening nozzles to achieve greater cavitation intensity requires higher flow rates, thus requiring pumps of higher capacities. In contrast, organ pipe geometry nozzles can enhance cavitation intensity without significant increase in pump capacity and have been used in deep sea drilling applications. The objective of this work is to study the effects of organ pipe inner jet nozzle geometry on co-flow water cavitation intensity and peening performance relative to a standard (unexcited) inner jet nozzle geometry through experiments on aluminum alloy Al 7075-T651. Nozzle performance is characterized via extended mass loss and strip curvature tests, high-speed visualization of the cavitation cloud, analysis of impulse pressures, and through-thickness residual stress measurements. It is found that the optimum organ pipe inner jet nozzle geometry enhances the mass loss and peak strip curvature by 61% and 66%, respectively, compared to the unexcited inner jet nozzle. Residual stress measurements show that the organ pipe inner jet nozzle produces deeper compressive residual stresses in the material than the unexcited inner jet nozzle.