'O God' in the Bible

Also I shook my lap, and said, So God shake out every man from his house, and from his labor, that performs not this promise, even thus be he shaken out, and emptied. And all the congregation said, Amen, and praised the LORD. And the people did according to this promise.

For they all made us afraid, saying, Their hands shall be weakened from the work, that it will not be done. Now therefore, O God, strengthen my hands.

For in the days of David and Asaph of old there was a leader of the singers, and songs of praise and thanksgiving unto God.