Here is how to Consume a list in SharePoint Online via Excel Online:
Open Excel > Data tab > From Other Sources > From Odata Feed
Important: You need to enter the location and call the List Data Service (you need to add _vti_bin/listdata.svc to the end):
Select Tables. In this example, we chose TestList
Modify the File Name: & Friendly Name: click Finish
Choose the radio button next to PivotTable Report > OK
Build the PivotTable and Save it to SharePoint Online.
When you open it, you will see the PivotTable
Now add to the SharePoint List
Go back to the Excel Online and choose Data > Refresh All Connections
You will now see the updated values in Excel Online!