OSAS: Is it the truth?

OSAS Protestants read the Bible with a certain set of assumptions, that cause them to read passages that others would take contradicting OSAS as not contradicting OSAS. If a passage says that those who do X will not be saved, an OSAS Protestant might say that such people are not Christians, or that such people might have appeared to be Christians but were never really saved to begin with, or that true Christians would never do X. I'll give one example from the beginning of Romans, maybe not the best example, but should be sufficient to demonstrate my point.

Romans 2
4Or despisest thou the riches of his goodness and forbearance and longsuffering; not knowing that the goodness of God leadeth thee to repentance?
5But after thy hardness and impenitent heart treasurest up unto thyself wrath against the day of wrath and revelation of the righteous judgment of God;
6Who will render to every man according to his deeds:
7To them who by patient continuance in well doing seek for glory and honour and immortality, eternal life:
8But unto them that are contentious, and do not obey the truth, but obey unrighteousness, indignation and wrath,
9Tribulation and anguish, upon every soul of man that doeth evil, of the Jew first, and also of the Gentile;
10But glory, honour, and peace, to every man that worketh good, to the Jew first, and also to the Gentile:
11For there is no respect of persons with God.

So, a Catholic might say that this passage shows God will judge every man according to deeds, and that immortal life is annexed to at least the avoidance of bad deeds, which seems to be the straightforward meaning. What will the OSAS Protestant say about verse 8 and on? What about those "that are contentious and do not obey the truth"? They might say that a truly saved person would never end up like that. Or they might say that this doesn't really apply to Christians because they know Christians are justified by faith and not by works. Or they might just say that the punishments mentioned are not hell. Or they might say something else, but whatever the case, their existing assumptions about OSAS cause them to read such passages in an OSAS way even when it's not the most straightforward reading.

Another problem, I imagine, is that many OSAS Protestants don't have a deep knowledge of Catholic doctrine and what they do know is a caricature. And on top of that, many view Catholicism with hostility so it is hard for them to think objectively and interpret things through a Catholic perspective.

If you haven't read through the entire New Testament at least I would do that. Even if you already have, you might want to re-read through the NT thinking about this question as you do, and noting down places you think might have bearing on the question, or that you would like to consider further. That would at least give you a starting reference for further study.

Do you have any particular places you wanted to discuss?

Look up Dispensationalism. Basically it's a protestant form of bible reading where the passages you agree with are for this age, and the ones that contradict you are obviously for a different dispensation.

OSAS is the result of trying to be neutral between with the predestinationism and Arminianism, while looking foolish to both parties.
Read Romans/John and try to understand what the author was originally stating then follow it.

So what did Jesus mean when he said that those who believe will "never be plucked from the hand of the lord" or that "believing on the lord gives everlasting life" ?

Then what did Paul mean in Galatians 3 and in Hebrews. Note that even Peter T O'Brien, a Calvinist accept that in Hebrews, the author or possibly Paul himself is giving a warning to some of his audience who are on the verge of falling away but nevertheless, can still be saved. Galatians 3 posits that Grace may not have an effect on people as that is what he says to them. Notices that he even mentioned that they had been Baptized by saying those that had been Baptized, put on Christ. He wont say that if his audience didnt receive it but he did. In the NT, Baptism is considered the initiation into the New Covenant and part of conversion, particularly in Acts. So the context and overall view argues against OSAS.

Jesus' statements in John can be explained by incongrous Grace and predestination. Note that so long as no one posits that one can indeed know one is Elect or is determinist, one need not proceed and say OSAS is true by that implication

The question of OSAS doesn't make sense.

The first is an allegory, which admits different interpretations. Suppose being in the Father's hand is the state of grace (i.e. being saved), one could reasonably interpret that statement as allowing you to voluntarily leave the Father's hand of your own will. The Father will not cast you out of his hand and no one else can pluck you out of the Father's hand, but that doesn't mean you can't leave by your own will. After all you can't "pluck" yourself. That kind of interpretation is given by St. Augustine in his Confessions, as I recall. Alternatively, one could interpret the verse in terms of predestination. In that case, those in the Father's hand are the elect, and it would be saying that God will infallibly guide those to salvation whom he has predestined to salvation.

I don't recognize the second example as any specific verse off the top of my head. However, that kind of statement, I would interpret in probably one of two ways. First, in Catholic doctrine, one "gets saved" through faith (in the sacrament of baptism, or through a baptism of desire), but one can lose salvation by falling into mortal sin. Or, depending on context, you might say that such a statement does not encompass everything necessary for salvation. For example, compare "an apple a day keeps the doctor away" with "an apple a day alone (all by itself) keeps the doctor away."

Yes very good.
There is more than this to consider however. Not only are there the statements in the Gospels by the Lord on this, there are also the statements after the gospels of assurance that they possess it. Now this isn't the same as saying that all certainty is true, but it is saying that there exists true certainty and they explain here not only that they have it but also what that true certainty is based on; it's based on the word of God— and that statement of assurance itself, is also itself in the word of God as I will quote below. If you regard these scriptures as inspired by God and infallible, then you are led to conclude they had correct certainty in their electness, and that certainty in turn was based in their confidence in the word of God. This certainty also happens to be the same faith which they believe had saved them, but only through God's grace in upholding his word, not by itself or by themselves. I hope that's not too confusing, I just made an attempt. Please consider the following.

1 John 5:13
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.

Philippians 1:6
Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ:

1 Corinthians 1:18
For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.

1 Peter 1:1-5
Peter, an apostle of Jesus Christ, to the strangers scattered throughout Pontus, Galatia, Cappadocia, Asia, and Bithynia,
Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you, and peace, be multiplied.
Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead,
To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you,
Who are kept by the power of God through faith unto salvation ready to be revealed in the last time.

Jude 1:1
Jude, the servant of Jesus Christ, and brother of James, to them that are sanctified by God the Father, and preserved in Jesus Christ, and called:

Hebrews 6:18
That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:

None of those verses prove your point right. As the Biblical view of Assurance is granted via the reliability of God, not men. OSAS and your view posits this is not the case and assurance is to be instead founded upon not the Word but instead on the "feeling" of epistemological certainty.

Yes
youtu.be/hycjHApNNOM
John 5:24
Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
you can't lose something that's everlasting and Jesus promises you shall never come into condemnation and that you are already passed from death to life.

Ephesians 1
13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.

Paul says that you're sealed with the Holy Ghost until the rapture, not until the next time you commit a big sin.

youtu.be/_EWnx5cTz_s