I'm unsure about this subject - I've heard both sides of the argument. 'Though if OSAS is true, I don't see how that squares with verses such as Heb 6:4, which states "It is impossible for those who were once enlightened...if they shall fall away, to renew themselves unto salvation, as they crucify the Lord afresh and expose Him to an open shame" and a similar statement by Paul, which says something like "If we sin wilfully after receiving God's grace, there is no (other way?) and results in a fearful expectation of wrath.
I think that the person described in verses 4,5 has been born again - he "has tasted of the Holy Ghost".
This saved person then falls away - I take this to mean he loses faith in Jesus - which means he is lost.
Verse 6 then says that it is impossible to renew this person to repentance while they are doing the actions of the two verbs (Greek - present participles) - that is - someone cannot be brought back to repentance while at the same time they are crucifying the Son of God and putting him to an open shame. (The Greek present participle is action occurring at the same time as the main verb)
The verse does not state (though it implies it) this, but it would seem that if the person stops the actions of the 2 verbs, then they can be renewed to repentance.