Magento Open Source, 1.9.x
One Page Checkout

The purpose of OnePage Checkout is to gather the information that is needed, and complete the sale as quickly as possible. When Onepage Checkout is enabled, the entire checkout process takes place on a single page. Each section of the checkout information is expanded as needed.

Magento’s Onepage Checkout is enabled by default. If you are implementing a custom integration or checkout extension, it might be necessary to disable Onepage Checkout.

To disable Onepage Checkout:

1. On the Admin menu, select System > Configuration.
2. In the panel on the left under Sales, select Checkout.

Checkout Options
3. Click to expand the Checkout Options section.
4. Set Enable Onepage Checkout to “No.”
5. When complete, click the Save Config button.