One Page Checkout (OPC) is a module that must be turned on in your CRE Loaded Admin. To turn it on you click on modules > Add-on Module and then select CRE One Page Checkout from the list and click INSTALL. That's all you have to do.
OPC works best when your customer is logged into your site. On the OPC page your customer will see Payment Information, Billing Address info, Payment Method, Product information and Shipping information (if applicable).
Depending on the payment process you as the Merchant use will directly impact how OPC works. If you use CRE Secure your customer will be redirected to our secure server through the HTML clone and will have to enter their payment details on the form that is created to look like your site. The credit card details are NOT added to the OPC page. Just like with PayPal or other types of payment processors, the payment is not completed on the OPC form.
Login or create an account is handled outside of the OPC process.