This is where I do worry about OSAS for that very reason you mentioned. I have a friend who believes in OSAS and by her actions, you'd really have to wonder if she was saved to begin with.
I worry about her because many who believe this way, don't think it's possible to not be saved no matter what.
My thoughts are that God knows those who are truly His. On the other end, we don't. We can know that if we are in Christ we're sealed with the Holy Spirit of promise.
But the person who is living a life of sin and believes that they're saved because of OSAS, it's very difficult to convince them that they might not even be saved. I like the word assurance of salvation over the OSAS terminology. Assurance of salvation is the more biblical term and I think it has more meaning.