The acronym OSAS should really stand for "Once a Son, Always a Son"
This is such a tattered, dog-eared topic of discussion that I hesitate to add to the stack of comments. Until now, though, I haven't yet encountered anyone bringing up the following simple text as proof of this point before. I am going to trust Christ's words themselves on this matter. He presented this topic in a basic metaphor to the Samaritan woman at Jacob's fountain. His words to her in John 4:13 (Interlinear) were as follows:
"...Everyone that drinks of this water will thirst again; but whoever may drink of the water which I will give him in no wise shall thirst for ever, but the water which I will give to him shall become in him a fountain (pege) of water springing up into life eternal."
This fountain (pege) is not just an ordinary well (phrear), a dug pit which fills up with water. A spring, or a fountain of living water is another thing altogether. Once Christ gives the gift of "living water" (John 4:10), the recipient is given eternal life in Christ which becomes an inexhaustible source of life within the person. Springs do not ever stop producing water, just as the source of life given to a child of God never ceases. To say that one can lose their status as a child of God (or walk away from it) would be to rip out Christ's metaphoric message from the John 10:12 context entirely.
We are not given a bucket of water that can be dumped when we once become a child of God - we are given an unending spring of living water within. As John 7:38 also says, "He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water." Can we "quench the Spirit" of God so that the flow of living water is slowed down? Surely. One can throw all kinds of junk into a spring to muddy the water, but the source of the pure, underground fountain is never changed.
Our adoption as sons of God is a covenant sealed by the unchanging Holy Spirit (Ephesians 1:13, 4:30). Sheep never turn into goats. Wheat never turns into tares. Sons never turn into bastards, even when these same sons need scourging or reproof. While sitting in the pigpen, the Prodigal Son was still a son, and still spoke of having a Father, even though the fellowship between them had been broken off by his behavior.
This is a doctrine that the child of God can truly REST upon. ("Come unto me all ye that labor and are heavy laden and I will give you REST...") That was the very purpose behind the picture of the Sabbath rest imposed on the children of Israel from ancient times. Why would God be so adamant that not a single act of labor be done on that day? It was to provide a picture for all time and all ages that the child of God has not done, and cannot do a single work to earn or to preserve their status as a child of God. This is why we have that one lone example of the man picking up sticks on the Sabbath that received the death penalty for it. You can't so much as pick up a stick to earn your way into the family of God, or to preserve your standing in the family. It's a God thing totally. We cannot take credit for the implanted spring within us, and we cannot get rid of this fountain once it is imparted.