Take a look at Deuteronomy 32:8-9 (King James Version):
"When the Most High divided their inheritance to the nations,
When He separated the sons of Adam,
He set the boundaries of the peoples
According to the number of the children of Israel.
For the LORD’s portion is His people;
Jacob is the place of His inheritance."
This is based on the traditional Hebrew text, otherwise known as the Masoretic text (MT). Now look at a much older version of this text that was found in the Dead Sea Scrolls and matches the Greek Septuagint text:
"When El Elyon gave to the nations their inheritance,
when he separated the sons of men,
he fixed the bounds of the peoples according to the number of the sons of God.
For Yahweh's portion is his people,
Jacob his allotted inheritance."
What on earth is going on here? The older text clearly shows that El Elyon is the father of all the gods, and Yahweh is simply one of his sons. El Elyon divides up the various nations and allocates a portion to Yahweh. The later version of the text simply merged these two gods into a single being.
There are other clues that there were originally multiple gods, take Psalm 82 for example, where Yahweh stands in the council of the gods.
1. Psalm of Asaph. God stands in the council of the gods; he judges among the gods. 2. How long will you judge unjustly, and show preference to the wicked? Selah. 3. Judge the poor and the orphans; do righteousness to the afflicted and dispossessed. 4. Deliver the poor and oppressed; save them from the hand of the evil. 5. They do not know and they have no understanding; they walk about in darkness. All the foundations of the earth are shaken. 6. I said, “You are gods, and children of Elyon, every one of you.” 7. But you will die like mortals, and fall like one of the princes. 8. Rise up, O God, and judge the earth, for you have inherited all the nations.
There are many other references to "other gods" in the Old Testament if you look carefully, have a look at my Bible Nonsense page for some examples. A lot of the rants by Yahweh in the Old Testament appear to be about his superiority over these other gods, presumably the ones El Elyon allocated to other lands.