I'm using One-Page Checkout 1.1.0 on ZenCart 1.55e and COWOA 2.7 beta, and Ceon Manual Card 4.01. Everything is working great with one hiccup that I believe is related to the One-Page Checkout addon
:
If a bad credit card number is entered (wrong number of digits), the normal prompt comes up about it. But after you put the correct one in, a note comes up 'Your order's details have changed. Please review the current values and re-submit.' (That text is in /includes/languages/english/checkout_one_confirmation.php)
Then you have to enter the credit card information AGAIN to get the order to process.

It is a pretty minor inconvenience that won't happen often but if there is an easy fix for it it would be nice if it never happened. Thanks for the great addon!