Is it possible that some use the term OSAS when they mean they believe in the security of the believer? I, for one, reject the doctrine of OSAS. I do not believe the Bible bears it out in either Old or New Testaments.
However, I fully believe and place my trust in the security of the believer (John 10:28,29, etc).
The difference is this: a believer is an obeyer, a doer of the word. So long as one abides in Christ through faithful obedience to Scripture, he has nothing to fear.
I think OSAS leaves itself open to some who might take advantage of their supposed indestructible state and engage in willful sin, all the while comforting themselves with the idea that, "Though what I'm doing is wrong, I'll still go to heaven because I was saved when I was younger..." or something like that.
Now I don't recall ever hearing such taught from a pulpit, (not first-hand at least), even by those who espouse OSAS, but I've seen many people whose actions said as much, and quite loudly. Some have even said as much when questioned about their actions.