The Old Testament actually has a lot to say about hamartology (i.e., the doctrine of sin), so I would like to explore some of the passages here for further reference.
Of course, I'll start with the Book of Genesis. The first sin we see is committed by Adam and Eve in the garden. Most of the known world is familiar with the story. The human couple choose to eat of the fruit at the prompting of another agent (i.e., the serpent) who tempts them with what appears to be an instrument/object/practice that God has placed in (or presented as something that should not be done) the garden as a test. The humans give into temptation and fail the test. Their offspring then enter into a continual battle with temptation and sin throughout the book. God makes numerous efforts to recreate the perfect environment in Genesis (e.g., the garden, the post-flood vineyard, the post-fall of Sodom and Gomorrah), but to no avail. The message is loud and clear: the problem is not the environment, but the humans themselves, who "is evil from his youth" (8:21). In fact, it is in this context that God declares that He will not wipe out everything on the earth again, i.e., attempt to destroy evil by destroying everything that lives, since the problem is man, not the environment itself. Hence, the reason God gives for evil in the world is man himself: literally stating that it is "because the formation of the heart/mind (i.e., inward man) is evil from his early life." Hence, it does not matter what environment God puts man into. He will find a way to corrupt it, and this is what we see throughout the book (and throughout the Bible for that matter).
This theme continues to run throughout the exodus and wilderness narratives, where even in God's salvific and fearful presence with Israel, after amazing miracles and theophanies appearing right in front of them, they are seen as remaining rebellious and sinful toward God. The same is true for once they enter the land filled with milk and honey, a Garden of Eden redivivus. God gives them the abundant land and subdues their outward influences, i.e., the corrupting influence of the Canaanites, but they remain rebellious. Surely, what David says of Nabal could be said of all man in its response to God, ""Surely in vain have I guarded all that this [man] has in the wilderness, so that nothing was missed of all that belonged to him; and he has returned me evil for good." (1 Sam 25:21).
In Job, we are told that man laps up iniquity like water: "Behold, He puts no trust in His holy ones, and the heavens are not pure in His sight; how much less one who is detestable and corrupt, man, who drinks iniquity like water!" (Job 15:15-16).
David remarks in the Psalms, "Behold I was brought forth in iniquity, and in sin my mother conceived me" (51:5). David here is lamenting over himself, and his own sin against God, so it is highly unlikely that he is saying his mother sinned in order to conceive him. Instead, it seems more likely in the context that he is talking about himself. What he says here is consistent with the rest of the Scripture. Man is sinful from early on in his life, and here we are told that the early time from which he is a sinner is from conception.
We are told in Jeremiah that man, therefore, must trust in the Lord and not in himself, since he is corrupt and his experience will lead him astray.
"Blessed is the man who trusts in the Lord And whose trust is the Lord. "For he will be like a tree planted by the water, That extends its roots by a stream And will not fear when the heat comes; But its leaves will be green, And it will not be anxious in a year of drought Nor cease to yield fruit. "The heart is more deceitful than all else And is desperately sick; Who can understand it? "I, the Lord, search the heart, I test the mind, Even to give to each man according to his ways, according to the results of his deeds. (17:7-10).
In Proverbs we are told that two different times that "there is a way that seems right to a man, but the end thereof is death" (14:12; 16:25). His corrupt heart and nature does not afford him enough insight to discern the right way to go. He, therefore, misreads the nature of reality with his distorted glasses.
But the question for us in all of this is, Why is man conceived in iniquity? Why is he corrupt from his youth? Why does he drink up evil like water, and rebel against God even to His face? In other words, from whence does this evil come?
In one sense, the Old Testament has already provided the answer in terms of understanding from whence it does not come. It does not come from the environment. It is not something produced by the environment. The Old Testament indicates that the environment merely provides further opportunity and temptation, but such would not stick if the man were not so attracted to it because of his lust for sin. Instead, it comes from the man himself. The problem is man. The problem, then, is human nature; but in what way is his nature corrupt? Is it an ontological corruption that leads to a spiritual one, or is it a spiritual problem that leads to ontological problems?
In some ways, this question is only explicitly answered in the New Testament and further developed by orthodox exegesis through the early Fathers who combated gnosticism, a heresy that posited the view that man was ontologically corrupt first, because he had been encased in the physical and finite by the evil Demiurge (the creator god who is seen as a devilish figure in gnostic thought), on through Augustine who argued extensively concerning the primacy of the spiritual, not ontological, corruption of man, down through the Reformers, who argued that man was completely corrupt spiritually, but not ontologically. Hence, God cannot be said to have created man as prone to sin in his ontological human nature, but instead man sins because of his rejection of God in the garden and the subsequent curse from God's salvific presence (not His omnipresence obviously) that would have given him his driving force to love what is good. Hence, sin is due to the corruption of man brought on by the Fall.
In another way, however, the Old Testament does provide us with this seed in terms of stating that man himself was not made evil. In fact, although Ecclesiastes is hotly debated in terms of its contrary statements toward the rest of Scripture, these contrary statements, in canonical context, serve as qualifications for understanding the whole of Scripture, specifically in the wisdom tradition. In 7:29, the Preacher states, "Behold, I have found only this, that God made men upright, but they have sought out many devices." In this statement we have both the confirmation that man is evil by his own choice, but that he was not made evil. Instead, he was made upright. What this does for us in providing context within the canon is to show us that the corruption that man has from his conception is not ontological. God didn't make him a sinful creature. Man is sinful by choice and his corruption must, therefore, be a spiritual one that stems from something else, and the only other "something else" in the Old Testament to which one can point is the removal of the human couple in Genesis 3 from God's paradise and the curse that is placed upon them because of their first sin.
It is, of course, possible to read this as God making man originally in the garden as upright, but that he sinned from there; but either way, the idea is that man, whether in the garden or in the womb, is not made ontologically prone to corruption. He has secured that for himself. If at creation, he has secured a cursed nature by sinning, and if in conception, he is working out an inclination toward sin from that curse. In fact, it is difficult not to conclude with the New Testament and orthodox Christianity, by way of inference, that man is born with a sin nature from the Fall. It's not like a baby can sin, so in what other way is David a sinner from conception, and yet created upright by God? It must be that his human nature is not sinful, but his fallen human nature, i.e., his disposition toward God at conception (i.e., spiritual disposition) is prone to sin.
In any case, what we have in the Old Testament are elements that are consistent with the New Testament and orthodox understanding of humanity. The Bible, as completed canon, is a strong countercultural witness, then, against the heresies of Gnosticism (finite human nature itself is corrupt or prone to corruption) and Pelagianism (humans are not spiritually corrupt or prone to corruption due to their disposition toward God created by the Fall/curse). The New Testament, and orthodox Christianity to follow, will simply draw out what is already there in seed form.