You can use OneNote Online to open, display, and edit existing pages created from a template. However, creating and customizing new page templates isn’t possible with OneNote Online.
OneNote Online is great if you’re looking to change text styles, apply text formatting, and use note tags to begin a custom page layout. You can then finish creating and saving it as a custom template in the OneNote desktop app on a Windows PC. For detailed steps about how to do this, see Create or customize a page template in OneNote 2016 for Windows.