"Once Saved, Always Saved" only applies to those that are truly saved. The Scripture is clear that not all who claim salvation posses salvation.
"What if someone says he is saved and then becomes a complete apostate, totally and permanently abandoning the faith and turning wholly to sin?", its not that he still is saved or that he lost his salvation, but rather that he never possessed it.
True children of God may backslide and become "carnal and walk as lost men" but when they do they will be convicted, chastened, and even called home, but they are still the children of God.
The distinction between the false professors and backslidden believers can often only be judged by God. However, it may sometimes be discerned by their fruit, their heresies, their hatred, their abandonment of the faith, their total return to their sin, or their lack of chastening for sin.