Whether all base are oxides?
NO, all bases are not oxides.
There are many examples of bases other than oxides, namely Ammonia(NH3),Potassium Cyanide(KCN)
Also, all oxides are not bases, there are also amphoteric and acidic bases.
One amphoteric base is Zinc Oxide(ZnO)
and a acidic oxide is Sulphur oxide(SO2)