To say that "Once saved, always saved" is a true doctrine is a little bit like saying that chewing one's food before swallowing helps one to live. It is true, but misleading. Chewing your food is not the point --ingesting food is.
"Once saved, always saved" leaves open the method and reason for salvation, and simply points out an irrelevant fact. OF COURSE a saved person will always be saved, but the reason a person is saved is because of God's choice --and God's reason for creating.
Whatever God sets out to do, he will indeed accomplish, and cannot be deterred from accomplishing it. Whatever does not happen, he never intended to happen. If God can be stopped from accomplishing what from the foundation of the earth he has begun, then he is not God.
But more than this, Salvation is simply God-made, God-done, God working in a person. It depends on God alone. His reason for creating is for his own sake, to show his power and greatness and beauty and glory, in that he has created for himself a dwelling place (and many other names describing the same result --his particular creation-- the Bride of Christ. This is not a set of random, nor of deserving, people drawn from a pool of possibles, but rather simply those he created for this particular purpose.
Again, it is important to understand this is for his sake, and not ours by comparison, though it is consumingly relevant as regards us, and that it does not depend on our decision. However, to say that works are not necessary, or that we do not decide, are also misleading. Works and the decision by the believer will inevitably occur, if a person belongs to Christ. As will perseverance, and obedience, and freedom from the slavery of sin, and so on --but these are the result of the change of heart that occurs as "regeneration" --these ARE the new creation, and not the cause of the new creation. We MUST obey; therefore, we do obey, and yes, it does take our effort and will, but this too is the work of God.
You see, God's economy is not our logistics.