I know this is hard to understand, but not believing in OSAS does NOT mean one believes that justification is by works. That is impossible. It is impossible to work yourself out of the debt of sin we have with God. IMPOSSIBLE.
What non-OSAS means is you are responsible to place your trust in God's forgiveness through the power of faith he gives to do that. Most people choose not to. A few do. A few of them continue in that trusting to the very end and are saved.
As long as your trust lies in Christ's finished work you have the assurance of your salvation and redemption on the Day of Wrath. Stop having trusting in Christ's finished work and you no longer have that assurance. The Bible is crystal clear about this.
You are trusting in your trust, rather than in the power of God to save. If you doubt for a moment, does that mean you lose your salvation. One only has to think of a child. Often if they get punished, maybe more than the siblings, they begin to think they are adopted. They feel that they are not the natural children of God. Yet, if they really thought about it, even adopted children are full children of the parents. And just because you doubt your parents love for you, doesn't mean they stop being your parents. Your thoughts and actions never stop your parents from being your parents.
To extend this metaphor a bit, I know I am a child of the King. Do you know how I know that?
"In love 5 he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will,6 to the praise of his glorious grace, with which he has blessed us in the Beloved. 7 In him we have redemption through his blood, the forgiveness of our trespasses, according to the riches of his grace, " Eph. 1:4b-7
"For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of
adoption as sons, by whom we cry, “Abba! Father!” Romans 8:15
"But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, 5 to redeem those who were under the law, so that we might receive adoption as sons. 6 And because you are sons, God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!”" Gal 3:4-6
I also know by these verses and many others that God has adopted me, I did not adopt or chose him. I don't trust in my trust, I trust in the mercy and sovereignty of God!