Office Online is getting communication enhancements thanks to Skype. Skype for Office Online comes as part of both Word and PowerPoint Online.
Skype for Office Online lives alongside your documents. It allows you to chat with co-authors as you work on a document in real time. You just click the Chat button to begin chatting. You can keep up with the conversation when you're away from the document from Skype on your mobile device or desktop.
While you only need your Microsoft account to use Skype with Office Online, Skype suggests that you link that account with your Skype account for the best experience, if you haven't already done so.