I will answer this question: If a Christian fails to obtain eternal life in the end I do not need to make a judgment call on whether he was "saved" at a point in time in the past or not, of if he was never saved. What I do know is that he did not believe in Jesus when he died and thus he did not have eternal life. (By the way, I do not believe in OSAS)
For a believer in OSAS, they must make the judgment call that such a person was never saved in the first place.
In that hypothetical, the Bible tells us they were not.
Once God calls us into his free gift of faith through his grace,seals us with his Holy Spirit and indwells us,as new creations in him,we are irrevocably his. And no one will take us from his hand.
We do not make a habit of Sinning. Because God knows us,and we are in him.
OSAS is the propaganda vehicle atheists manufactured long ago.
In order to discredit the Gospel by saying, when a Christian is Saved from Hell forever, they can Sin all they like.
In truth,their propaganda posited an unintelligent premise.
When you're eternally Saved from the penalty of Sin you can Sin all you like.
But,they were atheists. Enemies of God. Knowing nothing of the Gospel.
Their purpose was to disparage the truth of the teachings of the very God they first insisted doesn't exist.

Those were some Mensa members right there boy!
But what isn't funny is the fact so many Christians discount Eternal Salvation thinking that atheist slander , that it allows for Sin with impunity, is valid.
Which is why those self same self identified Christians insist on holding faith in their insecurity in Christ. Believing they have to work to stay right with God.
Sadly,those atheists made an impression.