It is true that once saved always saved but often our idea of salvation is that it is a one time past event. Trust in God is continual. Even in John 3:16 where it says "whosoever believes in Him" the actual tense of the verb (believes/trusts) in the Greek signifies not a one time past belief but a continual, present trust that doesn't end. There is no such thing as "once believed, now lost," or "I used to believe in God."
True continual trust in Christ will produce good works and God is the one who causes you to trust and causes you to produce good works.
Jesus said in John 15:5 "Apart from me you can do nothing."
"Now it is God who makes both us and you stand firm in Christ. He anointed us, set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come." -2 Corinthians 1:21
"It is God who works in you to will and to act according to his good purpose." -Philippians 2:13
Jesus said "All that the Father gives Me will come to Me, and the one who comes to Me I will by no means cast out." -John 6:37