Office Online Server (OOS) allows organizations to deliver browser-based versions of Word, PowerPoint, Excel and OneNote, among other capabilities. ... OOS is the successor to Office Web Apps Server 2013.

OOS scales well for your enterprise whether you have 100 employees or 100,000. The architecture enables one OOS farm to serve multiple SharePoint, Exchange and Skype for Business instances. OOS is designed to work with SharePoint Server 2016, Exchange Server 2016 and Skype for Business Server 2015. It is also backwards compatible with SharePoint Server 2013, Lync Server 2013 and, in some scenarios, with Exchange Server 2013. You can also integrate other products with OOS through our public APIs.

While OOS runs on your own servers, we are working hard to give you the same experiences you would get if you used our cloud service. For example, real-time co-authoring will now be enabled in Office Online Server. We will deliver most of the new Office Online features to OOS through regular updates.

Reference: