'O God' in the Bible

So do God to Abner, and so thereto: except that as the LORD hath sworn to David, I so do to him,

And when all the people came to eat meat with David, while it was yet day, David sware, saying, "So do God to me and so thereto, if I taste bread or ought else till the sun be down."

When David was come to the top of the mount and had bowed himself unto God: behold, Hushai the Archite came against him with his coat torn and earth upon his head.

And the king was moved and went up to a chamber over the gate and wept. And as he went thus he said, "My son Absalom, my son, my son, my son Absalom, would to God I had died for thee Absalom, my son, my son."