This issue arises when multiple CC (credit card) and/or CC save (Creditcard Save) methods are enabled but OSC is failing on validating user input data. The essence of this problem lies behind the way OneStepCheckout handles form fields as one big form vs Magento handles each payment method as separate form and this results form name collision in checkout page.

To solve this issue on default payment methods: * make sure that you are indeed using multiple CC methods and have form elements with same names in your page source * make sure which method causes you the trouble cc or ccsave or any other method with CC form fields * locate cc.phtml or ccsave.phtml from this base theme folder and copy to your template folder payment/cc.phtml or payment/cc_save.phtml , clear cache . If youhave some other method the process is the same locate the method form template and edit this

To solve this issue with custom payment methods colliding: * open up payment method form template and add following php code after $_code variable <?php $route = $this->getRequest()->getRequestedRouteName(); if($route == 'onestepcheckout'){

$fieldCode = '['.$_code.']';

} else {

$fieldCode = '';

}?> * replace all form names name="payment[... with name="payment<?php echo $fieldCode; ?>[..."