Is it true that the concept of original sin is nowhere to be found in the Hebrew Scriptures (OT), and that it was a concept originated by St. Augustine in the 4th (?) century? If this is the case it opens up a lot of questions.Indeed it would! Thankfully, it is not true.
We find, at least implicitly, references to original sin in the following verses from the OT:
Wis 2:24 but through the devil's envy death entered the world, and those who belong to his party experience it.
Job 14:1,4 Man that is born of a woman is of few days, and full of trouble. 4 Who can bring a clean thing out of an unclean? There is not one.
Psalm 51:5 Behold, I was brought forth in iniquity, and in sin did my mother conceive me.
Jer 17:9 The heart is deceitful above all things, and desperately corrupt; who can understand it?
Of course, we also have the action of Adam, an OT figure, whose sin against God brought original sin into the world (cf. Rom 5:12-21; 1 Cor 15:21-22).
As for St. Augustine being the inventor of the concept, we actually find reference to original sin before his time:
Irenaeus, Against Heresies,V:16:3, 180 AD:
And not by the aforesaid things alone has the Lord manifested Himself, but [He has done this] also by means of His passion. For doing away with [the effects of] that disobedience of man which had taken place at the beginning by the occasion of a tree, "He became obedient unto death, even the death of the cross;" rectifying that disobedience which had occurred by reason of a tree, through that obedience which was [wrought out] upon the tree [of the cross]. Now He would not have come to do away, by means of that same [image], the disobedience which had been incurred towards our Maker if He proclaimed another Father. But inasmuch as it was by these things that we disobeyed God, and did not give credit to His word, so was it also by these same that He brought in obedience and consent as respects His Word; by which things He clearly shows forth God Himself, whom indeed we had offended in the first Adam, when he did not perform His commandment. In the second Adam, however, we are reconciled, being made obedient even unto death. For we were debtors to none other but to Him whose commandment we had transgressed at the beginning.
Tertullian, On the Soul 40, 208 AD:
Every soul, then, by reason of its birth, has its nature in Adam until it is born again in Christ; moreover, it is unclean all the while that it remains without this regeneration; and because unclean, it is actively sinful, and suffuses even the flesh (by reason of their conjunction) with its own shame.
Origen, Commentaries on Rom 5:9, 248 AD:
The Church received from the apostles the tradition of giving baptism even to infants. The apostles, to whom were committed the secrets of the divine sacraments, knew there are in everyone innate strains of [original] sin, which must be washed away through water and the Spirit
Origen, Homilies on Lev 8:3, 248 AD:
Every soul that is born into flesh is soiled by the filth of wickedness and sin. . . . In the Church, baptism is given for the remission of sins, and, according to the usage of the Church, baptism is given even to infants. If there were nothing in infants which required the remission of sins and nothing in them pertinent to forgiveness, the grace of baptism would seem superfluous
Origen, Homilies on Jeremiah 8:1, 248 AD:
Everyone in the world falls prostrate under sin. And it is the Lord who sets up those who are cast down and who sustains all who are falling. In Adam all die, and thus the world prostrate and requires to be set up again, so that Christ all may be made to live.
Cyprian of Carthage, Letters 58:5, 253 AD:
But again, if even to the greatest sinners, and to those who had sinned much against God, when they subsequently believed, remission of sins is granted--and nobody is hindered from baptism and from grace--how much rather ought we to shrink from hindering an infant, who, being lately born, has not sinned, except in that, being born after the flesh according to Adam, he has contracted the contagion of the ancient death at its earliest birth, who approaches the more easily on this very account to the reception of the forgiveness of sins--that to him are remitted, not his own sins, but the sins of another.
These are just a few. Also see Original Sin and Original Sin in Tradition.