To enable Ipayment cc support in OneStepCheckout you need to :

  1. add ipayment references to onestepcheckoutindexindex in app/design/frontend/base/default/layout/onestepcheckout.xml. You can copy the references from app/design/frontend/base/default/layout/ipayment.xml and add them as follows by replacing (serach for to find the right place):

    One Step Checkout
    skin_jsonestepcheckout/js/window.js
    skin_jsonestepcheckout/js/onestepcheckout.js
    onestepcheckout/onestepcheckout.css

with


    One Step Checkout
     skin_jsonestepcheckout/js/window.js
     skin_jsonestepcheckout/js/onestepcheckout.js
    
    onestepcheckout/onestepcheckout.css
  1. next step is to add ajax event listener to checkout.phtml template to spy on ipayment ajax calls in app/design/frontend/base/default/template/onestepcheckout/checkout.phtml by replacing the following (search for !alreadyplacingorder to find the right place)
    /* Submit the form */
    $('onestepcheckout-form').submit();

with

    if(payment.currentMethod == 'ipayment_cc'){
        Ajax.Responders.register({
            onComplete: function(event) {
                if(event.parameters["payment[method]"]=='ipayment_cc'){
                    $('onestepcheckout-form').submit();
                }
            }
        });
        loaderelement.remove();
        submitelement.removeClassName('grey').addClassName('orange');
        submitelement.disabled = false;
        already_placing_order = false;
        payment.save();
        Event.stop(e);
    } else {
        /* Submit the form */
        $('onestepcheckout-form').submit();
    }