Let's walk through an example. Say you deposit 10 ETH with an average APY of 5%. First, you will notice that your wallet now has 10 ETH worth of oETH in it - these are your receipt tokens. When you go to withdraw your ETH after 1 year, you will trade back the oETH and receive 10.5 ETH in return (your original 10 ETH plus the 5% APY).