The main hub for all sizing related matters is ouraring.com/sizing. There you'll find more information about sizing options and how to find the right size. On that page you can also confirm the size you want. Here's an overview of how you can do that.
A.Confirming Your Size
A1. Go to ouraring.com/sizing
A2. Find the button that says Confirm Ring Size and tap it
A3. You'll be asked to give an email address. It should be the same address you used when you made the order. Tap Send Confirmation Link.
A4. An email will be sent to your inbox. It includes a link to a page on which you can see your order(s) and edit it by clicking the Edit link.
If you didn't get an email, please make sure that you used the same email address you made the order with.
A5. Choose your size.
A6. Press Save and Update Status. You can edit your size selection as long as your order status is Sizing or Shipping Queue.
B. Ordering an Oura Ring Sizing Kit
Follow steps A1–A4 specified above. When you get the confirmation email, tap the link.
B1. Choose the option Order a Sizing Kit.
B2. Press Save and Confirm. Please note that you cannot edit your sizing kit order after you've pressed Confirm. If you want to make changes to your sizing kit order after you've confirmed it, please contact firstname.lastname@example.org.
B3. Once you have determined your ring size with the Oura Ring Sizing Kit, go to ouraring.com/sizing again. Follow steps A1 – A6 on this page to confirm your size.