'O God' in the Bible

He shall not make himself unclean for his father, or for his mother, for his brother, or for his sister, when they die, because his separation to God is upon his head.

And Moses cried to LORD, saying, Heal her, O God, I beseech thee.

And they fell upon their faces, and said, O God, the God of the spirits of all flesh, shall one man sin, and will thou be angry with all the congregation?

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