'O God' in the Bible

But [in the meantime] the people were still sacrificing [to God] on the high places (hilltops) [as the pagans did to their idols], for there was no [permanent] house yet built for the Name of the Lord.

He said, “O Lord, the God of Israel, there is no God like You in heaven above or on earth below, who keeps the covenant and shows lovingkindness to Your servants who walk before You with all their heart;

Now, O God of Israel, please let Your word which You have spoken to Your servant David my father be confirmed.