Hi Federico,
Office Online in Office 365 is a part of SharePoint Online. To get started, sign in to Office 365 and create and save your document to SharePoint Online. Your organization’s plan give you libraries for storing your documents and OneDirve for Business for personal storage.
When you use the three-dot Open Menu in a SharePoint library, Office Online shows a preview of the document, and provides another Open Menu with commands for viewing and editing the document in your browser.
Thanks,
August