Microsoft has recently announced the ability for co-editors to chat with one another directly within a OneDrive document stored in OneDrive for Business or SharePoint Online when working in Office Online. Built on the same technology as Skype for Business, the new chat feature is available in all the Office Online applications—Word, PowerPoint, Excel and OneNote.
How to initiate chat?
When you share a document from OneDrive or SharePoint Online with your co-workers, they can view, make edits and even co-author with you in real-time. When multiple people edit the document at the same time, their names appear in the list of co-editors at the top right of the browser window. Next to the list of co-editors, there is a blue Chat button. (Shown below.)
Simply click on Chat button and send your message.
Chat history is not saved when you close the document but can be copied and pasted if desired. Use Comments (on the Review tab on the ribbon) when you want to attach a comment to a specific selection within the document, such as when you need to ask if a word or phrase should be changed. Comments are saved with the document and can be replied to, marked as done or deleted.
Source: Microsoft