The doctrine of OSAS is unbiblical. There are good evidences in the New Testament, that it is possible for a Christian to fall away from the Lord. The Sower parable is one. Also the book of Galatians is another. Another passage:
Acts 20:29-31
29 For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
30 Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.
31 Therefore watch, and remember, that by the space of three years I ceased not to warn every one night and day with tears
KJV
Falling away is a willful act - it isn't a helpless falling away. Always prayerfully living for Christ and pleasing Him in all things - needs to be kept at top priority. Any time a Christian allows himself to drift into lazy spirituality - he is in danger. The Laodiceans had fallen into lazy spirituality, and the result was bringing themselves under God's judgment.
Rev 3:16
16 So then because thou art lukewarm, and neither cold nor hot, I will spue thee out of my mouth.
KJV