Update: It looks like the extension is not quite ready yet, but it will likely be made available to Insiders in the near future.
According to the (translated) info from WindowsBlogItalia, the Office Online extension for Edge includes these features:
- Create a new document in Word Online, Excel Online, PowerPoint Online, OneNote Online, and Sway.
- Open a document from OneDrive or PC / tablet without the Microsoft Office suite is installed.
- Change the Microsoft Account with which you are logged.
- Or not to send feedback to improve the product.
Again, this extension is just for Windows 10 users on the Insider program who have downloaded the recent preview build 14342.