I honestly don’t know that much about OSAS. I know that it stands for Once Saved Always Saved, implying that the Christian can essentially do almost whatever they want and still not lose their salvation.
I don’t know when the anacronym started, probably sometime around or after the later part of the protestant reformation maybe. Either way, it seems as though some variation of this idea has been around for a while, particularly amongst the heretical gnostic Christians during the days of the apostles.
For example, Jude 1:4 says…
For certain men whose condemnation was written about long ago have secretly slipped in among you. They are godless men, who change the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord.
I think that many who hold to some form of OSAS do not necessarilly believe that they are denying the Lord Jesus Christ by believing that once they are saved they are always saved. And some adhere to it more than others do for sure. But, in effect, they do appear to be transforming the grace of our God into a license for immorality by saying that we can sin all we like to without losing our salvation in Christ.
In this sense, in my opinion, they are actually denying our only Lord and Savior, Christ Jesus, by saying that nothing matters except their faith which has saved them. As far as I can tell, they’re placing their own faith in God as being more important than the things God is actually calling them to do by the Holy Spirit.
It isn’t Scriptural as far as I can tell. It’s not a Catholic idea either-- and I think the idea has been condemned on numerous occasions by our Catholic Church.
Check out here for more information.