Why OSAS is true (amongst other reasons)...
Romans 5:10 (KJV) For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life.
This right here crossnote you show the misapplied teachings of the OSAS doctrine, as it pulls scriptures and makes them stand alone.
That is a faulty use of God's word as not even the whole chapter of Romans 5 can be taken without the rest of the epistle, especially that of Romans 6 that follows where Paul warns about allowing sin to continue to reign in our mortal bodies.
Paul is speaking to believers in 6 and tells them if they allow sin to continue to reign that sin is still their true master that will lead to death. The Greek word used for death here is not physical death, but means eternal death (spiritual).
Romans 6:21 (KJV) What fruit had ye then in those things whereof ye are now ashamed? for the end of those things is death.
That's only a description of how we lived.
Here's more context to Rom 5 if you need it...Romans 5:8-21 (KJV)
8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
9 Much more then, being now justified by his blood, we shall be saved from wrath through him.
10 For if, when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life.
11 And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement.
12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
13 (For until the law sin was in the world: but sin is not imputed when there is no law.
14 Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.
15 But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
16 And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.
17 For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)
18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.
20 Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound:
21 That as sin hath reigned unto death, even so might grace reign through righteousness unto eternal life by Jesus Christ our Lord.
Notice hoe GRACE REIGNS THROUGH THE RIGHTEOUSNESS OF CHRIST?