There are three legs upon which OSAS stands or falls. Do these verses mean what they supposedly mean, or have they been misunderstood and misapplied? When we examine them next to other scriptures that clearly contradict them will they hold up? Let's see.
First, 1 John 2:19—"They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us."
The OSAS interpretation of this verse says that those who "went out" were never in Christ to begin with; they were never saved. However, I suggest there's another meaning that conforms very well with other scriptures. This view says that those who "went out" were indeed saved and in Christ at one point, but were simply not as committed as the others. It's in this sense that they were not "of us." These are those spoken of in the parable of the sower (Matthew 13:5-6): "Other seeds fell on rocky ground, where they did not have much soil, and immediately they sprang up, since they had no depth of soil, but when the sun rose they were scorched. And since they had no root, they withered away." It sprang up, it had life to start with; but it "withered away."
Jesus' explanation of this passage is found in Matthew 13:20-21—"As for what was sown on rocky ground, this is the one who hears the word and immediately receives it with joy, yet he has no root in himself, but endures for a while, and when tribulation or persecution arises on account of the word, immediately he falls away."
Second is 1 John 3:6—"No one who abides in him keeps on sinning; no one who keeps on sinning has either seen him or known him."
This seems pretty cut and dried: if someone turns away back into sin they never knew Him. But we have to understand this in light of other scriptures that contradict it. Those who "never knew Him," never had a proper understanding of Him. This corresponds to the first group in the parable of the sower: "When anyone hears the word of the kingdom and does not understand it, the evil one comes and snatches away what has been sown in his heart." The seed was "sown in his heart," but was snatched away because of this person's lack of understanding.
Third is John 10:28—"I give them eternal life, and they will never perish, and no one will snatch them out of my hand." I agree with this verse: no one can indeed snatch them out of His hand. However, the person himself can walk away. For this reason I prefer the phrase "forfeit salvation" to "lose salvation."
Now let's look at some scriptures that plainly talk about walking away and falling away. I'm not going to use Hebrews because Hebrews is fiercely objected to by those who advocate OSAS, even though is has many relevant warnings about falling away. But Hebrews isn't necessary—there are plenty of others.
2 Peter 2:20-21—"For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first. For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them."
This passage clearly illustrates a person who was once in Christ but who turned their back on Him. This shows a conscious action on the part of the one turning back. These verses correspond with Luke 9:26: "Jesus said to him, 'No one who puts his hand to the plow and looks back is fit for the kingdom of God.' "
Matthew 24:10—"And then many will fall away and betray one another and hate one another."
1 Timothy 4:1—"Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons."
Matthew 24:13—"But the one who endures to the end will be saved."
Why say that those who endure to the end will be saved? If one cannot forfeit their salvation, why didn't He say "But all who have prayed the sinners prayer will be saved?"
Luke 9:26—"Jesus said to him, 'No one who puts his hand to the plow and looks back is fit for the kingdom of God.' "
Already mentioned, Luke 9:26 is a clear warning from the Lord about falling away.