Yes, OSAS is a pretty gross error, along with the doctrine of a strictly imputed righteousness. We can have a strong yet guarded level of assurance based on: God's goodness and trustworthiness together with evidence of fruit in our lives- just not 100% certainty. We're the wild card in it all so in light of human limitations, weaknesses, sinfulness, ignorance, etc, we should still, in humility, have a guarded assurance. And in real life I think we all know that intuitively anyway, and the average Protestant lives with at least some degree of healthy uncertainty if he's honest. God, alone, knows with perfect certainty whose names are written in the Book of Life and whose are not-while Scripture maintains that not all who think they're saved-are saved. As James affirms, and as Paul and John affirm in the passages I quoted, what we do counts. And any careful and objective reading of those passages can demonstrate that. But also all the other verses that instruct, exhort, encourage, warn, and admonish believers to be vigilant, remain in Christ, persevere, strive, do good, refrain from sin, live in the Spirit, etc. Otherwise, we can just ignore those if we prefer.
Another way to put all this: When God justifies man He really makes him just: forgiving, washing, cleansing, and making a new creation. Man, now under grace, in union with his Creator whom He was previously dead to (that relationship, itself, being the essence and basis of man's justice or righteousness), has the capability to become who he was created to be, to, for one, put to death sin, the deeds of the flesh, by the Spirit of God. But that action is an obligation, that we can still shirk even though it's absolutely impossible to achieve apart from Him. The obligation for man to be personally righteous in his thoughts and deeds does not go away with the New Covenant IOW, but now he's simply finally found the only authentic means to fulfill this obligation, by the power of God through communion with Him. Faith is the difference, the difference that makes that direct connection.
“I will put my law in their minds
and write it on their hearts.
I will be their God,
and they will be my people." 31:33
Adam thought he'd be better off going it alone. We're here to learn how wrong Adam was. Because:
"Apart from Me you can do nothing." John 15:5