Firstly, I'd like to thank all of you who took the time to answer my question. Much appreciated.
Secondly, it was interesting to read your responses and insights into what you considered "Original Sin" and why.
For my part I would say that I was originally inclined to agree with Mercury Rapids that 'disobedience' was THE original sin until I read this, Gen 3:13 "...And the woman said The serpent beguiled me, and I did eat." (KJV). The key phrase here being "serpent beguiled me" and also the key word being "beguiled".
In the first case, "serpent beguiled me", I'm inclined to think that the original sin was actually committed by the serpent who did the beguiling since the serpent knew that God did not want Adam and Eve to eat from the Tree of the Knowledge of Good and Evil. In this case it was the serpent who committed the original sin of 'disobedience to God's Will'. Even though the serpent wasn't directly commanded by God he knew God's Will concerning Adam and Eve (We know this from the conversation the serpent has with Eve.) and by deceiving Eve he was committing the sin of disobeying God's Will. However, since the serpent is not 'Man' ('Man' here means both man and woman.) this cannot be considered the original sin of 'Man'.
In the second case, "beguiled", I'm inclined to think that the original sin was that of 'disbelief in God's Word' i.e. Gen 2:17 "But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die." (KJV). If Eve had believed in God's Word on this matter she would not have been tempted/beguiled enough (or at all) to eat the fruit of the tree.
That's my take on it at this point in time.
What do you folks think about the above? (Looking for feedback please).