This is my take on OSAS.
When someone legitimately accepts Jesus as their Lord and Savior The Holy Spirit immediately indwells the person and seals them with the security deposit of their salvation.
Scripture says if anyone says they do not sin they are a liar.
8" If we say that we have no sin, we deceive ourselves, and the truth is not in us.
9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
10 If we say that we have not sinned, we make him a liar, and his word is not in us."
1 John 1:8-10.
So, we as believers all sin because of inherent sin nature.
But, as verse 9 says, of we confess our sins God extends His His Grace and forgives us.
There's a difference between deliberate sin and struggling with some and making effort to change. Repent means to change your mind.
The Holy Spirit in us helps us with sinful thought by convicting us and the flesh is contrary to the Spirit (Galatians 5:17)
so our spirit in coalition with The Holy Spirit moves us to obedience and we won't act on the sin as James describes.
14" but each person is tempted when they are dragged away by their own evil desire and enticed. 15 Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death."
James 1:14-16
A saved believer won't practice sin. The believer may sin, but The Holy Spirit moves the believer into repentance and the believer must obey the Spirit or the sin will continue into a practice of sin.
The saved believer won't lose salvation but will lose fellowship with The Lord and if they don't repent and are taken up to Heaven, then this unrepentant believer loses rewards.
This is an explanation for this in the Blue Letter Bible website by Don Stewart.
Can Sin Cause Us to Lose Our Heavenly Rewards?
Jesus said there's only one sin that's unforgivable.
28 “Assuredly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they may utter; 29 but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation”— 30 because they said, “He has an unclean spirit.”
Mark 3:28-30
Basically this is if a person were to curse God or abandon his faith and throw Jesus blood back at Him. This is unforgivable.
But a true believer won't do this because The Holy Spirit continually guides the believer towards the renewal of the mind and transformation into a new Creation like that of Christ.
A person who says they're a Christian but never grows in the renewal of the mind and leaves the faith Paul says this of them
"having a form of godliness but denying its power. And from such people turn away!"
2 Timothy 3:5
19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.
1 John 2:19
We need to remember Jesus spoke of Wheat and Tares that grow together for a while. But after a while we know who the true believers are because Jesus said of fake Christians, by their fruits you will know them .
Bottom line is a True believer cannot lose salvation, even for falling into temptation now and then but if they don't repent they lose fellowship with The Lord and lose rewards.
So it's best to repent than be put to shame when it's exposed
The fake Christian isn't legitimately saved and will ultimately be found out because they eventually gets exposed as a Tare.
11 And have no fellowship with the unfruitful works of darkness, but rather expose them. 12 For it is shameful even to speak of those things which are done by them in secret. 13 But all things that are exposed are made manifest by the light, for whatever makes manifest is light.
Ephesians 5:11-13