Salvation doesn't only consist of justification, that's where you fall short of understanding. Our predestination and election, calling and regeneration, justification and adoption, sanctification and mortification, glorification and completion of our redemption are all included in it. To say that believers can be justified but not sanctified, to be adopted but not persevere in the faith and in good works, is to call salvation an imperfect work. We must work out our salvation with fear and trembling, but even this is done by God in us to work and to will for his good pleasure. If you have been called and justified, God will carry on the work of perfecting you in holiness and all righteousness. He doesn't leave you to do the rest alone after you have been born again, his work carries throughout the Christian's life.
Yes, there are warning from God about endurance. Yes, they are meant to deter the believer from sin, but they are also indicators of who really is in the faith. However, those who are sensitive to such warnings will in effect, one way or another, immediately or some time later, heed such warnings that they will not remain in their sins. If we refuse to listen, God will chastise us and restore us to himself through the uses of disciplinary measures. His fatherly care for his children will be evident in their own lives, but most of all in their repentance. If they aren't, they prove themselves illegitimate children of God as the writer of Hebrews says.
It irritates me to hear people claim that OSAS teach that you can be lax about your obedience, when it is actually the opposite. Once saved always saved can actually be a fearful doctrine in my personal opinion, since it means that unless you bear fruit in keeping with repentance and continue in the faith, you were a liar to begin with. If that doesn't put fear in you to prove your election sure, then I would be concerned if you are even in the faith.
