That is an excellent point. OSAS is a flawed Protestant doctrine built upon the errors of Rome's Palegius heresy. Unfortunately, as you rightfully point out, many Protestants have fallen for this grievous error. I would point our Protestant brethren to the following confession:
Of the Perseverance of the Saints
I. They, whom God has accepted in His Beloved, effectually called, and sanctified by His Spirit, can neither totally nor finally fall away from the state of grace, but shall certainly persevere therein to the end, and be eternally saved.[1]
II. This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father;[2] upon the efficacy of the merit and intercession of Jesus Christ,[3] the abiding of the Spirit, and of the seed of God within them,[4] and the nature of the covenant of grace:[5] from all which arises also the certainty and infallibility thereof.[6]
III. Nevertheless, they may, through the temptations of Satan and of the world, the prevalency of corruption remaining in them, and the neglect of the means of their preservation, fall into grievous sins;[7] and, for a time, continue therein:[8] whereby they incur God's displeasure,[9] and grieve His Holy Spirit,[10] come to be deprived of some measure of their graces and comforts,[11] have their hearts hardened,[12] and their consciences wounded;[13] hurt and scandalize others,[14] and bring temporal judgments upon themselves.[15]
*Scripture footnotes at reference
Westminster Confession of Faith-Preseverance of the Saints
Most Protestants understand these principles although they have their doctrine skewed by years of listening to Rome's work based errors. Feeling people need to go to confession, take grace, do penitence, pray the Rosary, etc., is nothing more than reducing Christianity down to religion with ceremonial steps to achieve the goal of salvation.
This was certainly not the view given to us by Our Lord.
1) Catholics (and some Protestants) believe that once a person is saved they must maintain that gift given to them. There is a possibility that you could lose your salvation.
2) The "Once Saved Always Saved" Protestants look at salvation as a fix point of salvation. After that people are "free to choice" which way they want to go.
3) The "Perseverance of the Saints" Protestants believe that God saved you and God will keep you. And when you go off the beaten path, God will chasten and hasten you to either help you find your way or will call you home. But you will never be lost from the love of God who saved you in your most vilest state.
The first two views are based upon the egregious heresy of "free will"; one is capable of making correct and good choices. The last view is based upon an understanding that man with his sinful nature is wholly reliant upon the grace and mercy of God to guide us through this life.
It is an error and sin to think that man is ever capable of willfully choosing anything pleasing of God.