The custom display template weren't able to apply to cloud ssa content at on-premise search center

I did follow the guidance from https://technet.microsoft.com/EN-US/library/mt668455.aspx to add a Result Source to point to remote SPO. The content from on-premises and SPO are able to show up in my search center. I am not quite understand what you mentioned "Did you edit the Result Source to include external content in the Query Transform?". from the above document, it says keep the default setting in the Query Transform setting in Result Source setup.

"On the Add Result Source page, do the following:

1. In the General Information section, in the Name text box, type a name for the new result source—for example, Office 365 search index.

2. (Optional) In the General Information section, in the Description text box, type a description of the new result source.

   This description will appear as a tooltip when the pointer rests on the result source on certain configuration pages.

3. In the Protocol section, select Remote SharePoint.

4. In the Remote Service URL section, type the address of the root site collection inSharePoint Online that you want to get search results from, such as https://adventure-works.sharepoint.com.

5. In the Type section, select SharePoint Search Results.

6. In the Query Transform section, keep the default setting.

7. In the Credentials Information section, select Default Authentication.

8. Click OK to save the new result source."