Actually, early in Genesis we have people being called the sons of God.
Genesis 6:1–2 (NKJV): Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them, 2 that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose.
God calls Israel His son.
Exodus 4:21–23 (KJV 1900): 22 And thou shalt say unto Pharaoh, Thus saith the Lord, Israel is my son, even my firstborn: 23 And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.
Malachi 1:6 (KJV 1900): A son honoureth his father, and a servant his master:
If then I be a father, where is mine honour?
And if I be a master, where is my fear?
Saith the Lord of hosts unto you, O priests, that despise my name.
Psalm 2:7–8 (KJV 1900): 7 I will declare the decree:
The Lord hath said unto me,
Thou art my Son;
This day have I begotten thee.
8 Ask of me,
And I shall give thee the heathen for thine inheritance,
And the uttermost parts of the earth for thy possession.
I think the idea of God as a Father can be found in the OT