OSAS, in the modern context, is not true, which many take to mean that a person who made a public confession, and then goes off to renounce Christianity, is saved. Final perseverance is true, which states that the elect can never fall away, and only the elect can ever persevere, as the Grace of God is what differentiates them from others. An elect person might fall away for a time, but God always brings them back, and conforms them, through the sanctification of the Spirit, into the image of His Son. There is no treatise on 'Final perseverance." You are referring to the book I already quoted from, which is the treatise on the "gift" of perseverance. The word gift is significant.
Here's a second quote, just because I like to torment Papists:
"From a human perspective it is inscrutable why, given two pious men, one should be given the grace of final perseverance and the other not. From a divine perspective it must be the case that the individual who perseveres is among the predestined while the other is not. The one who fails to persevere has not been called according to God's plan and chosen in Christ according to God's purpose." (Augustine, Treatise on the Gift of Perseverance)
I believe the Papists look as Augustine as the crazy Church Father in the attic.