Most people think that air stones are physically pumping air into an aquarium, while they do increase oxygen, it is not because of this.
The keyword here is surface agitation. The air stone is simply pushing water to the surface where it receives oxygen. As it bubbles away its continuously pushing new water to the surface.
While an air stone may be adequate, a spray bar could potentially be more effective. Running along the length of your tank just below the surface pointing slightly upwards, as the outflow of your filter will ripple the surface of your water, over a much larger area. A long spray bar could nearly cover the entire surface allowing lots of oxygen exchange.
Spray bars are very efficient and most filters can adapt to these, however you could make your own by just attaching a spray bar to the filters outflow, or simply having the outflow pipe, just below the surface, creating surface ripples. The more rippling, the more oxygen.
If you only have an air pump, you could attach the air-line hose to a air stone spray bar that will create a bubble wall and therefore create a large surface area rather than a focused point, as an air stone would.