Office Online (OO), known as Office Web Apps (OWA) on SharePoint 2013, describes the light-weight, browser-based versions of Microsoft Office. Generally, this term refers to the online versions Word, Excel, PowerPoint, and OneNote (the Big Four), though it also includes Outlook, Project, and Visio.
Office Web Apps (OWA) is sometimes confused with Outlook Web App (OWA); they are not the same thing. Outlook Web App is part of Office Web Apps.
Office Web Apps was introduced with SharePoint 2013 and was installed with the SharePoint environment but on its own server. SharePoint 2013 did not require OWA, though in many cases organizations that upgraded to SharePoint 2013 also installed an OWA server and the applications. Though consumer versions of OWA were available as early as 2010 via Skydrive (the predecessor to consumer OneDrive), OWA did not become available to business users of SharePoint and OD4B until SharePoint 2013 was released. OWA was rebranded Office Online in Office 365 in 2014 and eventually future versions of server-based OWA would become known as Office Online Server.
When you click a Microsoft proprietary file (e.g., .docx, .xlsx, .pptx, .one), the file will open in the appropriate browser app in read mode. PDF files open in Word Online by default, though this can be changed.
You can also access OO directly through the waffle or the Office 365 home screen.
Features and functionality
The Big Four in OO are generally useful because loading files is faster in OO than opening the client application for the app in question. If a file is simply clicked in SharePoint or OneDrive for Business, it opens in a read mode and the app provides the option to move into editing mode either in the online app or the client app.
OO apps are supported in Edge, Internet Explorer, Chrome, and Firefox. While Safari is not supported by Microsoft, usability testing has shown OO works correctly in Safari.
In SharePoint 2013, 2016, and Online, Office Online can be disabled at the library level by going to
Library Settings > Advanced Settings > Open in the client application (must be a Site Owner to perform this operation).