Original Sin is the Result of Man using his free will and choosing to turn away from God.
From the Catechism:
Man let let his trust in his Creator die in his heart and, abusing his freedom, disobeyed God’s command. In that sin man preferred himself to God and by that very act scorned him. He chose himself over and against God. in this act Adam and Eve immediately lost the grace of original holiness.
They became afraid of the God of whom they had conceived a distorted image - that of a God jealous of his prerogatives. The control of the soul’s spiritual faculties over the body was shattered; the union of man and woman became subject to tensions, their relations henceforth marked by lust and domination. Harmony with creation was broken: visible creation has become alien and hostile to man.
Because of man, creation is now subject “to its bondage to decay”. Finally, the consequence explicitly foretold for this disobedience will come true: man will “return to the ground”, for out of it he was taken. Death makes its entrance into human history. (Cf. catechism Para 385 ff.)