The OTO, or One Time Offer, is an upsell. This is generally an upgrade of the original offer that a customer purchases. After creating your upsell as a standalone product you will need to put it into the OTO/upsell field within your funnel.
There are two different buttons for the product once you have set it up. The first set will be for the original product. Scroll down and you'll see the buttons for your OTO. These are the buttons that will be displayed on your OTO sales page. Your customers will be taken to the OTO page after paying for the original product. They will be given the option to purchase the OTO, if they decide not to purchase they will be taken back to their original purchase.