Mycroft, the only people who are eternally saved are those who endure to the end (Matthew 24:13). Jesus was not addressing groups of people but individuals when He said this. How does one "endure" to the end? By God's grace through faith in Jesus Christ. Once we are saved we must continue to live by faith, not by sight or our own selfish desires.
I used Troy Perry and Mel White as examples of men who started their journey in Christ as men of faith in God's truth and then exchanged faith in God's truth for a false theology that allows them to live openly sinful lives. These men have put their faith in their own interpretaton of God's word and as a result have been cut off and cannot be grafted back into the natural olive. To maintain that OSAS applies to them is to label evil good.
You interpret Romans 11 to mean groups of people but not all Jews were cut off and not all gentiles are grafted into the natural olive. If this were the case then none of those saved on the day of Pentecost were saved since they were all Jews.
1 Timothy 4:1-5
4:1 But the Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons, 2 by means of the hypocrisy of liars seared in their own conscience as with a branding iron, 3 men who forbid marriage and advocate abstaining from foods, which God has created to be gratefully shared in by those who believe and know the truth. 4 For everything created by God is good, and nothing is to be rejected, if it is received with gratitude; 5 for it is sanctified by means of the word of God and prayer. NAS