I believe in OSAS, and believe there are many scriptures to support this. At the same time, there are scriptures which could be interpreted to say a person can lose salvation. This passage is one example. Anybody who denies this does not know the scriptures very well.
So anybody that does not agree with you simple does not know the scriptures? And you believe the scriptures contradict themselves as you believe they teach both OSAS and deny OSAS and so God is the author of confusion.
Have you ever heard of the hermeneutic principle that the unclear, the ambigous ought to be interpeted by the clear and unambiguous. You admit "here are scriptures which COULD BE INTERPRETED TO SAY" which means there is some room to wiggle.
John 6:37-39 and other such clear and unambiguous scriptures (Jn. 5:24; 10:26-28; etc.) teach OSAS without ambiguity.
Romans 11 is without question concerning THE ELECT NATION of Israel whereas Israel is set in contrast to the elect "remnant." You confuse them in your argument against OSAS. The remnant is the CURRENT elect that has already obtained salvation at the present time. They are NEVER said to have been cut off and NEVER said to have to be grafted back in again - that is OSAS.
In contrast to the elect "remnant" there is Elect Israel as a nation that has been cut off and will be grafted back in "again." They were in unbelief, they are in unbelief but because of unconditional election they will not remain in unbelief but will be grafted back again but presently that is not the case as they presently are "enemies of the gospel FOR YOUR SAKE" (Rom. 11:25-28). You are attempting to make Israel as a nation (not the PRESENT remnant) the basis of your argument against OSAS when in fact Israel has a nation has NEVER BEEN SAVED to this date. As a nation they NEVER believed. They are cutt off for "unbelief" in the sense that the WHOLE NATION rejected Jesus Christ which was consistent with their prior state of unbelief and treatment of God's prophets but now comes to a climax of unbelief in rejecting the Messiah. They were ALWAYS IN UNBELIEF but never to this extent that they are now "enemies of the gospel." So your example fails as any example used against OSAS must first be saved, must first be in belief but Israel NEVER has been saved, NEVER has been in belief always unbelief but never unbelief to this extent. Likewise the Gentiles as nations have never been saved always in unbelief. However, when they come to the extent of unbelief so that they admantly reject the gospel entirely as did Israel they too will be cut off and God will return to UNBELIEIVING Israel who will be brought to faith by unconditonal election as spelled out in Romans. 11:28 - enemies turned to faith.
So your whole analogy against OSAS is false. Israel NEVER was in beleif and so it is impossible to use an UNBELIEVER falling from faith to disprove OSAS.
So their cutting off and grafting in has nothing to do with election or salvation because nations are not saved and lost and then saved again and neither are individuals. The cutting off and grafting in has to do with all the EXTERNAL priviledges and promises obtained by being the focus of God's sphere of redemptive activity and those promises are listed in Romans 3:1; 9:5 which are found in the root (promises of Abraham, Isaac and Jacob) or those things presently given to the church in Matthew 16:19, 18:18 - kingdom privileges (authority, house of God, oridinances, ministry, etc.).
The house of God, ordinances, ministries, etc. are now among the Gentiles as God's PRIMARY sphere of redemptive activity in calling out his elect which is the manifestation of FAITH from among the Gentiles just as previously the house of God, ordinances, ministries, etc. had been among Israel because Israel had been the PRIMARY sphere of redemptive activing in calling out his elect which the manfistation of FAITH from among Israe. When all the elect come in from among the Gentiles they no longer will stand "by faith" but will be cut off as God had cut off Israel.
There is nothing about individual salvation in this cutting off and grafting in again.
This passage concerns election, being part of "The Branch" which is Jesus (Zech 6:12). Jesus is "the Elect One". The only reason we are elect is because when we believe we are baptized into his body.
Being chosen in him was not made at the point of faith but before the world began (Eph. 1:4) and election is UNTO salvation not because of salvation (2 Thes. 2:13). You are simply ignoring the cause and consequence points in these texts as well as the time elements and that is what you must do to deny what these texts actually say and teach.