I belong to a Southern Baptist church, and they teach OSAS. The explanation for someone falling away later, possibly renouncing their beliefs entirely, is that "they were never saved to begin with." I for one don't feel qualified to make that call. I know a lot of atheists and agnostics who once professed Christianity, and I know it would hurt them very much if someone were to tell them they were never Christian in the first place if they are not one now.
Yet God is not fickle. He's not going to disown His children so easily. Being human, we're still going to fall into sin even after Jesus Christ is Lord of our lives. That's the old adage about how we're not perfect, only forgiven. It's not about what we do, it's what He did, and all that. I acknowledge the concern I've seen voiced, that taken to the extreme, OSAS means a person can ask Jesus Christ into his or her life, become saved, then go out and commit a mass murder, or some other heinous crime, and still be saved. My response would be that most likely, someone who has taken Jesus Christ as Lord of his/her life would do no such thing afterward, except possibly in the case of severe dementia. Then it's God's call.
Do I agree with my church teaching that once saved, we can never lose it? Sorta. I don't believe God is going to disown me if I am confessing my sins to Him, and one slips my mind and I forget to mention it. I don't believe God is going to turn me away if I'm still struggling with a particular sin when I die. For example, I'm an alcoholic who has been sober since 2008. That was a relapse. I had come to sobriety long before then. Let's say I happen to relapse again, as any alcoholic could, and my intoxication results in an accident that kills me. Is God going to turn me away because He cares more about "going by the book," so to speak, than about my relationship with Him? I don't think so. Do we disown our children when they do something bad? No, they're still our children.
But I do believe it's possible--not likely, but possible--for someone who once believed to stop doing so, and to renounce his or her salvation. It that situation, I'd say it's less a case of "losing" your salvation, and more case of deliberately giving it up. My pastor would probably say the person who did so was not saved in the first place. I don't know one way or the other.