I believe in OSAS, but this doctrine is better understood in my opinon as the eternal perseverance of the saints. Once Saved, a person becomes sealed into the family of God, and he is always saved, under the power of the Holy Spirit and the Grace of the Father keeping them Saved. However, as we know, it takes more than just a mere verbal prayer or verbal profession of faith to receive the forgiveness of Sin and Salvation from Sin.
Let me post just a couple of verses that I believe teach the eternal security of believers.
John 10:25-30
25 Jesus answered them, "I told you, and you do not believe. The works that I do in My Father's name, they bear witness of Me.
26 But you do not believe, because you are not of My sheep, as I said to you.
27 My sheep hear My voice, and I know them, and they follow Me.
28 And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.
29 My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father's hand.
30 I and My Father are one."
And I also love what the Apostle Paul says in 2 Timothy 1:6-12
6 Therefore I remind you to stir up the gift of God which is in you through the laying on of my hands.
7 For God has not given us a spirit of fear, but of power and of love and of a sound mind.
8 Therefore do not be ashamed of the testimony of our Lord, nor of me His prisoner, but share with me in the sufferings for the gospel according to the power of God,
9 who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was given to us in Christ Jesus before time began,
10 but has now been revealed by the appearing of our Savior Jesus Christ, who has abolished death and brought life and immortality to light through the gospel,
11 to which I was appointed a preacher, an apostle, and a teacher of the Gentiles.
12 For this reason I also suffer these things; nevertheless I am not ashamed, for I know whom I have believed and am persuaded that He is able to keep what I have committed to Him until that Day.
There are indeed many other passages of Scripture that teach about the eternal security of the believer and that one whom is truly born-again cannot lose his salvation. However, I believe that the two passages I posted are extremely clear in supporting the doctrine of OSAS or the eternal perseverance of the saints.