We are excited to announce the release of Office Online Server (OOS), which allows organizations to deliver browser-based versions of Word, PowerPoint, Excel and OneNote, among other capabilities. Microsoft recognizes that many organizations still value running server products on-premises for a variety of reasons. With OOS, you get the same functionality we offer with Office Online in your own datacenter. OOS is the successor to Office Web Apps Server 2013.
Read more at source: https://blogs.office.com/2016/05/04/office-online-server-now-available/