Original Sin is the idea that Adam and Eve sinned against the Lord in the Garden of Eden and as a consequence all humans are born into sin. They read scriptures like Romans 5:12-21 to mean Adam was the head of all of humanity and when they sinned he made the choice for every human being that would live after him. Some scholars talk about the sinful nature within humanity and believe that Christians have this as well.
Original Sin is not in the Bible. It is a doctrine of human beings to attempt to understand how the sin of the first humans plays out in the lives of every human being sense. So also, the sinful nature is also not in the Bible. When Paul uses this type of terminology he refers to the old man or the old life. Christians do not have a sinful nature because they have been made pure and holy Saints of God. They can be affected by this sinful world around them and tempted to sin, but that is a choice they have to make. Otherwise, why would we constantly be told to live holy and righteous if it was not possible? This would create a Christian with two competing natures within him. Paul has already told us in Galatians 5 that the Spirit desires was contrary to sin and sin what is contrary to the Spirit. We are dead to sin and alive in Christ.
I believe a better understanding is that Adam and Eve with their sin opened up the door to evil in the perfect world. This created an environment of sin, death and decay all around us. We are born into a world that carries the environment that makes human beings prone to sin or propensity toward sin. We tend to have a bent toward sin before we meet Christ.
In my opinion, a better way to understand Adam's sin is that it set in motion since entrance into this environment to destroy it and us. We are born into an environment that makes it extremely easy to give in to sin and to continue getting into sin. We always have a choice when it comes to sin after we come to know Christ and serve him. He gives us freedom from sin (Romans 6). We are also exposed to sin by watching everyone around us. So by environment and example sin is a regular occurrence in our life.