I did some digging into this and found that the "form-control" class is not hard coded onto fields, but added via js when the page is initially loaded.

$('input[type="text"], input[type="password"], input[type="email"], input[type="tel"], textarea, input#state').addClass('form-control');
$('select').addClass('select_caret_icon form-control');

My guess is I have to re add these after OPC's AJAX handler has been fired. Any pointer in which file this needs to take place?