A One Page (sometimes called One Step) Checkout implementation on eCommerce websites reduces the number of steps in conversion. It shortens the funnel, increases conversion and reduces cart abandonment. When we replace standard checkout with OPC, we loose functionality to save abandoned carts as prospects. These prospects can be 'incentivized' later to complete transaction. It is recommended to implement save prospect on minimum data entered in tandem with One Page Checkout. We have written a blog article on it which explains OPC in even more detail.
We are eCommerce/Magento implementation experts and have consultants who can help you implement features like this that will impact your topline.