Tacitus tells us that the supreme god of the Germans was called Esus or Hesus, and though some have thought he meant the Celtic Hu, it is far more likely that he had heard the word As or Æs, the favorite Teutonic term for the divinities. 
     The word is known in all the Teutonic languages:  it is As, Aasir in the North, Os, Es in Anglo-Saxon, and Anseis or Ensi in Gothic and High German. 
     The Aasir are in northern myth a family like the Olympian gods of Greece; they inhabit Valhalla, and there receive the spirits of the worthy dead, to feast and hunt with them till the general battle and final ruin of all things, when a new and perfect world shall arise.
     Asa appears in the Landnama-bok, and Aasir, the collective term for the gods, is used in Norway as a name corrupted into Asser, or Ozer.  It is probably the same with Esa, the ancestor of the Bernician kings, who may have used 'Os' in compliment to him. (History of Christian Names, Yonge, 1884)