The OTO, or One Time Offer, is an upsell. This is generally an upgrade of the original offer that a customer purchases. So you create your upsell and put that in the OTO field.
You will get two different buttons for the product once you set it up. The first set will be for the original product. Then 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.