It is not the gospel.
This is the gospel:
1 Corinthians 15:3-4
[3] For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; [4] And that he was buried, and that he rose again the third day according to the scriptures
Romans 1:16-18
[16] For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. [17] For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith. [18] For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness
Paul did not say the phrase “Once Saved, Always Saved” anywhere in his letters.
In fact, one could deduce that a OSAS believer could, with a good conscience, hold the truth in unrighteousness; because the OSAS doctrine allows for it.