Babying it too much! Too much water and fertilizer. Oregano (and other mediterranean herbs) have best flavor when they are kept pretty dry and in lean soil. It concentrates the essences. Only water it when it is really dry. Don't fertilize.
Other thoughts- did you let it bloom? If it starts making flowers and seeds, the leaves lose their flavor. Have you been trimming it back? If you just let it grow, it gets woody and tends to lose flavor.