The original sin was simple disobedience to the command of the Creator.
Genesis 2:16-17 "...the LORD God commanded the man...of the tree of the knowledge of good and evil thou shalt not eat: for in the day that thou eatest thereof thou shalt surely die."
The occasion of the disobedience was brought about by the deceiver. He first cast doubt upon the Word of God: Genesis 3:1 "The serpent...said unto the woman, Yea, hath God said...?" He then flatly denies God's Word: Genesis 2:4 "Ye shall not surely die..." Eve was decieved, agreeing with the deceiver, and ate of the tree that God had commanded them not to eat of.
That is the original sin.
The Word of the Creator is the "logos", God's thought/plan/purpose/blueprint of creation. God spoke all things into being, and upholds all things by the "word of his power" (Hebrews 1:3).
Sin is the missing of the mark, the falling short of God's plan or will. Righteousness is believing God; Romans 4:3.