Q: The God of the O.T. seems so different then the Father of the N.T.?
A: God is mentioned as a ‘Father’ in a number of OT scriptures, particularly by David in the Psalms and by several prophets. See: ICh 17:13, Ps 2:7, Is 64:8, Mal 1:6, 2:10 to name a few. Nevertheless, the picture of God portrayed in the OT is substantially different. Only David seems to have a real Father/Son relationship with God, and that is a foreshadow of the relationship between Jesus and his Father.
The Book of Job is recognized as perhaps the oldest book of the Bible (though Genesis and other books record earlier events and traditions) and specifically deals with the question of the relationship between God and his creation including humankind. God is presented here as quite remote and beyond our comprehension.
It is in the teaching of Jesus, and only through him that we have any relationship with God as The Father. Remember, without Christ, he really is not our Father and we have no relationship with him, we can only hope for the best and see if we can work our way into his good graces—that was the OT and the OT law. Without Christ, all you get is judgment with only a hope of mercy and justice. God actually owes us no explanations–that is the point of the Book of Job. He blesses who he wills and gives judgment upon who he chooses.
Praise God for his Son Jesus and the Good News. Through Christ we can be adopted children of God and He becomes Our Father. I’ll take it!