One of the most exciting Power BI Report Server features introduced in 2017 was the ability to publish and view Excel workbooks on the report server portal. Although Power BI Desktop is increasingly the preferred choice for data analysis, Excel workbooks and particularly Excel connections to SQL Server Analysis Services (SSAS) models are still very important to many business users as well. Having both Excel and Power BI in the same portal, in addition to paginated reports (.RDLs) gives BI teams the flexibility to meet many different needs from the same solution. 

The Office Online Server (OOS) must be installed to enable Excel online functionality in the report server. The full process and technical details for deploying the Office Online Server is available at MS DOCs via the following URL: https://docs.microsoft.com/en-us/power-bi/report-server/excel-oos.

From a Power BI Report Server configuration perspective, the Office Online Server Discovery Endpoint URL must be added within the Advanced properties of Site settings as shown in the following screenshot:

This URL is the InternalUrl used when deploying the OOS server, followed by /hosting/discovery. As described at the beginning of this section, Site settings (Gear icon | Site settings) can only be accessed by users assigned to the System Administrator role.