My comments are in brackets.
Matthew 26:31
Then Jesus said to them [followers of Jesus - disciples], "You will all fall away because of Me this night, for it is written, ' I WILL STRIKE DOWN THE SHEPHERD, AND THE SHEEP OF THE FLOCK SHALL BE SCATTERED.'
Matthew 24:10
10"At that time many will fall away - [implies this is refering to believers since one cannot fall from a position if he was never in that position in the first place] - and will betray one another and hate one another.
Luke 8:11-15
11"Now the parable is this: the seed is the word of God.
12"Those beside the road are those who have heard; then the devil comes and takes away the word from their heart, so that they will not believe and be saved.
13"Those on the rocky soil are those who, when they hear, receive the word with joy; and these have no firm root; they believe for a while, and in time of temptation fall away. - [saying they "believe" implies that these are people who do become believers (Christians) but it does not last since it says they "fall away"] -
14"The seed which fell among the thorns, these are the ones who have heard, and as they go on their way they are choked with worries and riches and pleasures of this life, and bring no fruit to maturity.
15"But the seed in the good soil, these are the ones who have heard the word in an honest and good heart, and hold it fast - [this verse is obviously talking about those that become Christians - why would this say that they "hold it ('the word') fast" if it would be impossible for them not to do this? - if they are eternally secure no matter what, then why even say anything about them "holding it fast"?] - and bear fruit with perseverance.
1 Timothy 4:1
But the Spirit explicitly says that in later times some will fall away from the faith, - [addressing believers (Christians) since these people were obviously in the "faith" as they could not have fell from it otherwise] - paying attention to deceitful spirits and doctrines of demons,
Hebrews 3:12
Take care, brethren - [again, refering to believers], - that there not be in any one of you an evil, unbelieving heart that falls away from the living God. - [why warn these people of the possibility of them getting an evil, unbelieving heart and of falling away if it would be impossible for them to do so?]
Hebrews 6:4-6
4For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit,
5and have tasted the good word of God and the powers of the age to come,
6and then have fallen away - [addressing believers and says that they can fall away], - it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame.
2 Peter 3:17
You therefore, beloved, knowing this beforehand, be on your guard so that you are not carried away by the error of unprincipled men and fall from your own steadfastness, - [context implies that believers are being addressed - they cannot fall from their own steadfastness unless they are in a position to fall from steadfastness in the first place]
Galatians 5:18-22
18But if you are led by the Spirit, you are not under the Law.
19Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality,
20idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions,
21envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God. - [why is it assumed that a person that once professed faith in Jesus as Saviour is exempt from this warning?]
1 Corinthians 6:8-10
8On the contrary, you yourselves wrong and defraud. You do this even to your brethren.
9Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals,
10nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God. - [again, why is it assumed that if one that once professed faith in Jesus Christ, fell into these types of sins, he would still inherit the kingdom of God?]
Revelation 21:7-9
7"He who overcomes will inherit these things - [seems to be implying that one that is a believer could possibly not overcome] -, and I will be his God and he will be My son.
8"But for the cowardly and unbelieving and abominable and murderers and immoral persons and sorcerers and idolaters and all liars, their part will be in the lake that burns with fire and brimstone, which is the second death."
Hebrews 3
1Therefore, holy brethren, partakers of a heavenly calling, - [referencing believers] - consider Jesus, the Apostle and High Priest of our confession;
2He was faithful to Him who appointed Him, as Moses also was in all His house.
3For He has been counted worthy of more glory than Moses, by just so much as the builder of the house has more honor than the house.
4For every house is built by someone, but the builder of all things is God.
5Now Moses was faithful in all His house as a servant, for a testimony of those things which were to be spoken later;
6but Christ was faithful as a Son over His house--whose house we are, if we hold fast our confidence and the boast of our hope firm until the end.
7Therefore, just as the Holy Spirit says,
"TODAY IF YOU HEAR HIS VOICE,
8DO NOT HARDEN YOUR HEARTS AS WHEN THEY PROVOKED ME,
AS IN THE DAY OF TRIAL IN THE WILDERNESS,
9WHERE YOUR FATHERS TRIED Me BY TESTING Me,
AND SAW MY WORKS FOR FORTY YEARS.
10"THEREFORE I WAS ANGRY WITH THIS GENERATION,
AND SAID, 'THEY ALWAYS GO ASTRAY IN THEIR HEART,
AND THEY DID NOT KNOW MY WAYS';
11AS I SWORE IN MY WRATH,
'THEY SHALL NOT ENTER MY REST.'"
12Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the living God. - [this verse is addressing brethen (believers) and says that falling away is a possibility]
13But encourage one another day after day, as long as it is still called "Today," so that none of you will be hardened by the deceitfulness of sin.
14For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end, - [the word, "if" , means that is possible that a believer could not hold fast in which case, as this says, the person will no longer be a partaker of Christ] - 15while it is said,
"TODAY IF YOU HEAR HIS VOICE,
DO NOT HARDEN YOUR HEARTS, AS WHEN THEY PROVOKED ME."
16For who provoked Him when they had heard? Indeed, did not all those who came out of Egypt led by Moses?
17And with whom was He angry for forty years? Was it not with those who sinned, whose bodies fell in the wilderness?
18And to whom did He swear that they would not enter His rest, but to those who were disobedient?
19So we see that they were not able to enter because of unbelief. - [this is comparing a Christain's walk with Jesus to the Jew's walk with God in the wilderness - because of their unbelief they were denied access to His rest - it also equates their disobedience to their unbelief - this seems then to be implying that if a believer is disobedient and has unbelief (or that disobedience would be a sign that the person would be in unbelief), he will not be able to enter into God's rest which seems like a reference to eternity with God]
I have no doubt that some that believe in OSAS will probably find a way to try to explain these verses in the context of OSAS. However, the people that believe in conditional security would probably do the same with verses presented by those that believe in OSAS to support that doctrine. I think that there are verses that could be used to support either doctrine. However, I believe that the doctrine that seems to flow more easily from the Bible is that of conditional security. In other words, I think one has to try harder to make verses "fit" the doctrine of OSAS than one does for the belief of conditional security.