I am very uneasy about OSAS. There is a sense in which it is true, but the 'P' in Tulip stands for the Perseverance of the Saints, which IMO is much better and more Biblical (Matthew 24:13).
The Lord Jesus said,
"But you do not believe because you are not My sheep, as I said to you. My sheep hear My voice, and I know them, and they follow Me. I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand" (John 10:26-28).
So first of all, one does not become a sheep by believing; one believes because one is a sheep.
Secondly, Christ's sheep are absolutely secure.
Thirdly, Christ's sheep are a special breed. They are distinguished by their ears and their feet. They
hear the shepherd's voice and they
follow Him. Both verbs are in the Present Tense, which in Greek denotes continuity. They are hearing the shepherd's voice and they are, on a continuing basis, following Him. So if someone had professed faith at some point in the past, but was not now following Him or hearing His voice (whether read or preached), on what basis would he suppose that he might be a sheep? Would he not be more likely to hear the Shepherd speak the words of Matthew 7:21-23? Christ's sheep He knows from eternity; others He does not know at all. 1 Corinthians 10:12.