OSAS is the belief that once one has made a confession of faith in Christ, one is saved, regardless of subsequent behaviour or repentance (contra Matt. 7:21-23; Luke 6:46).
The Perseverance of the saints is altogether more Biblical. Here are three articles from the 1646 Baptist Confession. It's a bit less wordy here than the better-known 1689 Confession. Note especially Article XXIX.
XXIII.
All those that have this precious faith wrought in them by the Spirit, can never finally nor totally fall away; seeing the gifts of God are without repentance; so that He still begets and nourisheth in them faith, repentance, love, joy, hope, and all the graces of the Spirit unto immortality; and though many storms and floods arise, and beat against them, yet they shall never be able to take them off that foundation and rock, which by faith they are fastened upon; not withstanding, through unbelief, and the temptations of Satan, the sensible sight of this light and love, be clouded and overwhelmed for a time; yet God is still the same, and they shall be sure to be kept by the power of God unto salvation, where they shall enjoy their purchased possession, they being engraven upon the palms of His hands, and their names having been written in the book of life from all eternity.
Matt. 7:24,25; John 13:10, 10:28,29; 1 Pet. 1:4,5,6; Isa. 49:13,14,15,16.
XXVI.
The same power that converts to faith in Christ, carrieth on the soul through all duties, temptations, conflicts, sufferings; and whatsoever a believer is, he is by grace, and is carried on in all obedience and temptations by the same.
1 Pet. 1:5, 2 Cor. 12:9, 1 Cor. 15:10; Phil. 2:12, 13; John 15:5; Gal. 2:19,20.
XXIX.
All believers are a holy and sanctified people, and that sanctification is a spiritual grace of the new covenant, and an effect of the love of God manifested in the soul, whereby the believer presseth after a heavenly and evangelical obedience to all the commands, which Christ as head and king in His new covenant hath prescribed to them.
1 Cor. 12; 1 Pet. 2:9; Eph. 1:4; 1 John 4:16; Matt. 28:20.
If a professing Christian continues indefinitely in sin and/or unbelief, what evidence can there be that he has been given New Birth (2 Cor. 13:5)?