The OT's only mention of Adam outside of Genesis is in the genealogy in 1 Chronicles. Nowhere does it ever connect the Israelites' acts of disobedience with Adam's sin, as Paul does. [Source]Well, today I'm here to confess to you that I was wrong. It's somewhat embarrassing to realize that a point I have made so many times on this blog, thinking it was a real zinger, but it won't do to keep saying it now that I know that it is just flat-out wrong. Specifically, because of Isaiah 43:27 (which doesn't mention Adam by name but makes his identity fairly clear): "Your first father sinned, and your mediators transgressed against me."
The link between Adam's sin and the human condition is made only by Paul. Jesus never mentions Adam; he is only mentioned in the Old Testament as part of a genealogy in 1 Chronicles 1:1, never as an explanation or ground for anyone's sin. [Source]
Let's not take it too far: this verse does mention Adam's sin and connects it to Israel's subsequent unfaithfulness (in the midst of a polemic beautifully contrasting Israel's waywardness with the Lord's faithfulness to redeem them and forgive their sins), but still doesn't come close to articulating anything that could be considered an orthodox Christian "doctrine of original sin". Adam seems more like a prototype or example of sin that his children follow after; there is no mention of sin "entering the world", much less an "Adamic covenant". Still, it is certainly something.
And given what I've been learning since making some of these sweeping statements about Adam, I can't say I'm surprised. The line between what we regard as divinely revealed doctrine and things that were simply part of the cultural air that the Biblical authors breathed is very fine indeed. For example, in Genesis 1-2, the idea that the world was created from chaos by a deity was common sense in the Ancient Near East; the important thing in the story is how God does it, not that He did it. Similarly, the Israelites' ideas of heaven, hell, angels, demons, and other things may have developed considerably during their rule by Persia due to the influence of Zoroastrianism. Of course, this intermixing of revelation and "culture" goes both ways; an idea's originating outside the Bible (particularly the Protestant Bible) does not preclude it from being true or "revealed".
So this verse in Isaiah might be a glimpse at an embryonic form of the doctrine of "the Fall" that Paul brings up in 1 Corinthians 15 and Romans 5. Rather than Paul arriving at it in a vacuum, it presumably developed and gained popularity during the Second Temple period, explaining how Paul can mention it relatively offhandedly to his audiences.