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If you use the "click to open" OneNote in Windows, you can create Outlook Tasks directly from the notes you are taking. These tasks will then show up in Outlook or O365 Tasks. If you are in a meeting you can easily save Tasks and complete them later. Unfortunately there is no way to create an Outlook Task in OneNote for Office365 Web.
How to:
1. Open "click to open" OneNote for Windows.
2. Begin take notes.
3. Highlight the text that you would like to turn into an Outlook Task.
4. Click on the menu bar that pops up. You will see a flag icon which will allow you to choose from various options for your task.
5. Open up your favorite Task program like Outlook or Microsoft To-Do to view your newly created task along with your other tasks.
For more information see this Microsoft article.