OSAS is an extremist view and not held by many Protestants in reality (even those who say they do, are incorrect to the true meaning of what they are purporting and under further examination find that they indeed do not believe in strict OSAS theology)
It is taking a principle tooooooo far and is erroneous
Just as Calvinism takes God’s sovereignty and foreknowledge, etc too far and has everyone predestined for heaven or hell at birth
To be honest, I don’t think even in my Protestant days I knew one person that held to OSAS in the extreme view.
Here are some examples of what scripture says on this:
I Jn 2:18 "Children, it is the last hour! As you have heard that antichrist is coming, so now many antichrists have come. From this we know that it is the last hour. 19They went out from us, but they did not belong to us; for if they had belonged to us, they would have remained with us. But by going out they made it plain that none of them belongs to us. 20But you have been anointed by the Holy One, and all of you have knowledge. 21I write to you, not because you do not know the truth, but because you know it, and you know that no lie comes from the truth. 22Who is the liar but the one who denies that Jesus is the Christ?This is the antichrist, the one who denies the Father and the Son. 23No one who denies the Son has the Father; everyone who confesses the Son has the Father also. 24Let what you heard from the beginning abide in you. If what you heard from the beginning abides in you, then you will abide in the Son and in the Father. 25And this is what he has promised us, eternal life.
26 I write these things to you concerning those who would deceive you. 27As for you, the anointing that you received from him abides in you, and so you do not need anyone to teach you. But as his anointing teaches you about all things, and is true and is not a lie, and just as it has taught you, abide in him".
Luke 8:11 ‘Now the parable is this: The seed is the word of God. 12The ones on the path are those who have heard; then the devil comes and takes away the word from their hearts, so that they may not believe and be saved. 13The ones on the rock are those who, when they hear the word, receive it with joy. But these have no root; they believe only for a while and in a time of testing fall away. 14As for what fell among the thorns, these are the ones who hear; but as they go on their way, they are choked by the cares and riches and pleasures of life, and their fruit does not mature. 15But as for that in the good soil, these are the ones who, when they hear the word, hold it fast in an honest and good heart, and bear fruit with patient endurance."
as you can see some DO fall away - there had to be something to fall away from! You can lose your salvation by your own doing - by rejecting what you know is true!
Deception and lack of endurance, etc are reasons why people fall away - but as I John states this does not have to be the case if we abide in truth
remember: the race we run is a Marathon, not a sprint
I Cor 9: 24 Do you not know that in a race the runners all compete, but only one receives the prize? Run in such a way that you may win it. 25Athletes exercise self-control in all things; they do it to receive a perishable garland, but we an imperishable one
Hebrews 12Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight and the sin that clings so closely,and let us run with perseverance the race that is set before us, 2looking to Jesus the pioneer and perfecter of our faith, who for the sake of the joy that was set before him endured the cross, disregarding its shame, and has taken his seat at the right hand of the throne of God.