What was the original sin? Partaking of the forbidden fruit, or choosing to go against God's wishes and do so?
Adam and Eve's original sin was eating the forbidden fruit, not deciding to (go against YHWH's direction and) eat the fruit.
The fact is plainly recorded in Genesis. YHWH Himself says that He is punishing the man and the woman for eating the forbidden fruit, not for deciding to eat the forbidden fruit. It's not as though YHWH beamed down as soon as Eve decided to eat the fruit and punished her for making that decision. Instead, it's the act for which she (and Adam) are punished.
Furthermore, YHWH denied them the tree of life not because they'd disobeyed, per se, but because they had become wise. Not wanting them to become, like Him, both wise and immortal, he drove them away from the tree of life. It was eating the fruit, not deciding to eat the fruit, that made them wise.
As modern people, we place great stock in intents rather than in deeds. It's easy to think of a mental decision as a sin, while something physical and basic (like eating a fruit) doesn't seem to qualify. We're likely to say that a man who has sex with his mother out of ignorance isn't guilty of committing the sin of incest. But for ancient people it was deeds that counted, not intent. Just ask Oedipus.
Part of the reason we moderns concern ourselves with intent is the direction given to Western civilization by Jesus of Nazareth. He's the one who said that lust is adultery and hate is assault. But the story of YHWH and the Garden of Eden predate Jesus both chronologically and culturally.
Alternative Answer: It was YHWH lying about the tree of the knowledge of good and evil that is the original sin. Without that lie, the serpent would not have been able to use the truth to induce Eve to eat the fruit. Maybe if YHWH had been straight with Adam and Eve, the whole forbidden fruit thing wouldn't have happened. He could have said, "Don't eat this fruit because if you do it will make you wise, and then I'll have to doom you to keep you down, plus I'll curse you in the bargain."