If it were simply 'proof texts' which upheld OSAS than Luther and Calvin wouldn't have been able to make such elaborate theologies of the belief. They were extremely well versed in the Scriptures, in rank with any Catholic bishop.
OSAS is between the lines of Scripture, and laced in it's essence- they held, simply, that God does not make mistakes. If one goes in and out of salvation, than God is errant in His salvific merits to men.
Calvinists have have outright proclaimed that TULIP is the actual gospel of Christianity altogether, and it's not hard to see why- it's extremely well thought theology. They didn't receive such a grand notion through proof texting.
It may be a well thought out theology, but it's not what the Scriptures teach. As you said, it's between the lines'. Since one has to read between the lines to see it tells us that its not in the Scriptures. Luther and Calvin were both influenced by Gnosticism which they got from Augustine. When Augustine developed his "Perseverance of the Saints" it was rejected by the church. You said Luther and Calvin were well versed in the Scriptures, that doesn't mean they understood them.
Everyone who approaches the Scriptures does so with a world view. Luther and Calvin had a world view. Their world view was Catholic. While they rejected some of the errors of the Catholic church, they retained some also. In addition to the errors they kept they introduced their own, one of which is OSAS. As I've said there is not a single passage of Scripture in all 66 books that says salvation can't be lost. Do you believe a doctrine of this magnitude would go without mention anywhere in Scripture? I said before that this doctrine was reject by the early church as heresy and those who taught it were the Gnostics. The apostle John didn't have good things to say about the Gnostics.
Anyone who is going to be intellectually honest has to ask themselves how this doctrine could be rejected by the church for 1400 years and yet be what the Scriptures teach. As you said, it is between the lines, that means it's not there. It exists by way of inference. Christians infer it from passage of Scripture. The reason it is inferred is because it's not there.
Here is a quote from Origen which was before the year 300 AD. Note his opposition to what appears to be today's Calvinism.
8. Let us begin, then, with what is said about Pharaoh—that he was hardened by God, that he might not send away the people; along with which will be examined also the statement of the apostle, “Therefore hath He mercy on whom He will have mercy, and whom He will He hardeneth.” And
certain of those who hold different opinions misuse these passages, themselves also almost destroying free-will by introducing ruined natures incapable of salvation, and others saved which it is impossible can be lost; and Pharaoh, they say, as being of a ruined nature, is therefore hardened by God, who has mercy upon the spiritual, but hardens the earthly.
Notice he says those who say, man has no free will, a ruined nature incapable of salvation, and cannot be lost, are misusing the passages. All three of those tenets are part of Calvinism.
The doctrine isn't in the Scriptures, was rejected by the church for 1400 years and still is rejected by the church, was argued against and declared heresy by the earliest Christians. Do you really suppose that Luther and Calvin knew something that the previous millions of Christians didn't? I don't think so. They let their presuppositions and world view color their view of Scripture. Instead of trying to build a theology strictly from the Scriptures the simply modified their existing theological systems which already contained errors.
