Throughout Scripture, God insists on a co-operative effort between Him and man.
And ...
He chose us, and we must choose to co-operate with our sanctification.
This is where OSAS falls.
There is ELECTION, and there are free-will CHOICES to be made!
And these choices are in co-operating with our sanctification, or not co-operating.
Or there are other options ... like walking away, falling away, etc.
You don't think believers have ever walked away from the faith?
If not, why all of the many warnings about it?
.
when God saves us, its forever and eternally saved, we are in progrssive sauntification while here on earth, and will be fully perfected when receive glorified forms!