The use of plasma assistance is shown to enhance the optoelectronic properties (i.e., transparency, free carrier density, and conductivity) of indium tin oxide (ITO) deposited by reactive magnetron sputtering by promoting the incorporation of oxygen in substoichiometric oxide films during magnetron sputtering. The authors demonstrate that subplantation of oxygen ions (O-2+ and O+), i.e., their implantation to depths of several nanometers below the growth surface, is the primary pathway by which radio frequency plasma assistance at the substrate surface enhances oxygen incorporation during reactive magnetron sputtering of ITO. These conclusions are supported independently by elastic recoil detection measurements of elastic recoil detection measurements of ITO films in the time-of-flight regime and Monte Carlo TRIDYN simulations of oxygen ion bombardment in the reactive low-pressure plasma environment. The findings indicate that subplantation plays a crucial role in improving the optoelectronic properties of O-deficient ITO films.