Comparision of donation power
...According to my opinion it should be Oxygen as it is more electronegative than Nitrogen...


It's correct that oxygen is more electronegative than nitrogen. However, this makes an oxygen group a poorer electron donating group than the corresponding nitrogen group. Electronegativity means that the atom is less willing to give up the electrons. Nitrogen is more willing to give up (donate) its electrons because it is less electronegative.