'O God' in the Bible

And Moses, crying to the Lord, said, Let my prayer come before you, O God, and make her well.

Then falling down on their faces they said, O God, the God of the spirits of all flesh, because of one man's sin will your wrath be moved against all the people?

And Balaam said to God, Balak, the son of Zippor, king of Moab, has sent them to me, saying,