My contrib uses some of its own routines, which is why it probably is not compatible with the One Page Checkout contrib. My contrib targeted some routines in osC that I felt were lacking and improved them. I am not familiar with the One Page Checkout contrib. If I had more time I could troubleshoot this for you, but I don't right now. Sorry I can't be more help, but it was designed to work with the standard osC checkout system.