Using a buffer layer under the anode, we studied outcoupling enhancement for organic light-emitting diodes. Monolayers of tungsten oxide and titanium oxide were used as the anode buffer layers. Thin anode buffers such as 25nm and 50nm tungsten-oxide layers, and a 15nm titanium-oxide layer noticeably increased the outcoupling. However, thick buffers decreased the outcoupling. The combined effects of the constructive interference and the creation of additional bound modes explain these behaviors. Therefore, the thickness of the anode buffer layers should be optimized under the upper limit, which creates additional bound modes.