Our Father...

Jesus often referred to God as "father."   He began His model prayer with "Our Father which art in Heaven" (Matt 6:9 KJV)— Πατερ ημων ο εν τοις ουρανοιςn — Pater emon o en tois ouranois.  His use of the word father in reference to God is so frequent in comparison to Old Testament usage that we might get the impression that it is his own distinctive idiom.  Although it is not a common mode of expression for God in the Old Testament, it does exist.

The phrase our father occurs two times in the Septuagint clearly referring to God:

Isa 63:16 KJV – Doubtless thou art our father (ημων ει πατηρ – emon ei pater / אבינו – avinu), though Abraham be ignorant of us, and Israel acknowledge us not: thou, O LORD, art our father (πατηρ ημων – pater emon / אבינו – avinu), our redeemer; thy name is from everlasting. 

Isa 64:8 KJV – But now, O LORD, thou art our father (πατηρ ημων – pater emon /  אבינו – avinu); we are the clay, and thou our potter; and we all are the work of thy hand. 

The phrase our father apparently is referring to God in the Septuagint translation of 1 Chronicles 29:10; the punctuation suggests it in the Greek edition I am using (Ευλογητος ει, κυριε ο θεος Ισραηλ, ο πατηρ ημων – Eulogetos ei, kyrie o theos Israel, o pater emon) and in the Septuagint translations to English.

1 Chr 29:10 NETS – And Dauid blessed the Lord before the assembly, saying, "Blessed are you, Lord God of Israel, our father (ο πατηρ ημων – o pater emon / אבינו – avinu) from age even unto age."

1 Chr 29:10 Brenton – And king David blessed the Lord before the congregation, saying, Blessed art thou, O Lord God of Israel, our Father (ο πατηρ ημων – o pater emon / אבינו – avinu), from everlasting and to everlasting. 

Translations of 1 Chronicles 29:10 from Hebrew are ambiguous or clearly tilt toward "God of our father Israel" (NIV).

Other Old Testament texts that speak of God as a father include:

Ps 68:5 KJV – A father (אבי – avi) of the fatherless, and a judge of the widows, is God in his holy habitation. 

Ps 89:26 KJV – He shall cry unto me, Thou art my father (אבי – avi), my God, and the rock of my salvation. 

Prov 3:12 KJV – For whom the LORD loveth he correcteth; even as a father (וכאב – ukh'av) the son in whom he delighteth. 

Isa 9:6 KJV – For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father (אביעד – avi'ad), The Prince of Peace. 

Jer 3:19 KJV – But I said, How shall I put thee among the children, and give thee a pleasant land, a goodly heritage of the hosts of nations? and I said, Thou shalt call me, My father (אבי – avi); and shalt not turn away from me. 

Jer 31:9 KJV – They shall come with weeping, and with supplications will I lead them: I will cause them to walk by the rivers of waters in a straight way, wherein they shall not stumble: for I am a father to (לאב – l'av) Israel, and Ephraim is my firstborn. 

Mal 2:10 KJV – Have we not all one father (אב – av)? hath not one God created us? why do we deal treacherously every man against his brother, by profaning the covenant of our fathers? 

The apocryphal book Tobit refers to God as a father:

Tob 13:4 Brenton Septuagint – There declare his greatness, and extol him before all the living: for he is our Lord, and he is the God our Father (πατηρ ημων – pater emon) for ever. 

I especially like the text in Psalm 89.  (I have encountered this Psalm several times recently in other studies.  The word emunah (אמונה – faithfulness) occurs seven times in it— Ps 89:1–2, 5, 8, 24, 33, 49.)

Palms 89:26
He shall cry unto me,
Thou art my father, my God,
and the rock of my salvation.