You do not understand OSAS.
OSAS does indeed say that sin can separate God and man, in fellowship, which is why repentance is necessary.
I knew this about OSAS way back since I studied it 7 years ago. This is not new for me. I am 100% aware that OSAS teaches that sin is only a loss of fellowship and yet it is not a loss of salvation. But the Bible teaches that...
Believers cannot be out of fellowship with the Lord and be saved.
#1. 1 John 5:12 says He that has the Son has life and He that does not have the Son does not have life. Life is associated with eternal life or salvation.
#2. John 17:3 says eternal life is in knowing the one true God, Jesus Christ. Knowing implies a fellowship. So if you don't know Jesus, then you don't have life (Salvation).
#3. Romans 8:9 says if he a man does not have the Spirit of Christ, he does not belong to him.
#4. Psalms 73:27 says God will destroy all those who abandon Him (or go a whoring from Him).
#5. John 15:6 says if a man does not abide in Him, he is cast forth and burned.
#6. 1 John 1:7 says if we walk in the Light as He is in the Light, the blood of Jesus Christ cleanses us from all sin. Being cleansed of sin is salvation; If you were to contrast this with 1 John 2:4, it says that person who says they know the Lord and does keep His commandments is a liar and the truth is not in them. Revelation 21:8 says all liars will have their part in the Lake of Fire.
#7. Romans 11:21-22 says if you do not continue in his goodness you will be cut off. For if God spared not the natural branches (i.e. the Jews), take heed that he can do the same to you (i.e. Gentile believers). The analogy here is that you are branch and Christ is the tree. We need to continue in Christ's righteousness or goodness, not our own righteousness or goodness, or we will be cut off because of unbelief.
#8. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates? (2 Corinthians 13:5).
You said:
We do not believe that sin can affect salvation because it is not our justification it is Christ's justification that is in our place.
Hebrews 10:26 says if we willfully sin after we receive the knowledge of the truth, there remains no more sacrifice for sin.
1 John 1:7 essentially says we have to walk in the light in order for the blood of Jesus Christ to cleanse us from all sin.
Hebrews 5:9 essentially says that Jesus is the author of eternal salvation to all who obey Him.
If you can show Christ sinned or that His life was not perfect then that would affect Christ's perfect justification or being our Advocate to the Father.
Where did I ever state that? I didn't. So you are proposing something that I never said and nor do I believe.
You said:
I wish you understood OSAS because correcting your misunderstanding is tiring.
God's grace teaches us to deny ungodliness and that we should live righteously and godly in this present world (Titus 2:11-12). God's grace is not a license for immorality (Jude 1:4). Hence, why King David was not saved in his sins of adultery and murder. David needed to confess of his sins to be forgiven. This is what we read about in Psalms 51. David cries out for forgiveness with God and desires for his salvation back.