There is a managed property named `SiteTitle` , but this property is not `refinable` and therefore it makes it impossible to use in `Refinement` web part.

![enter image description here][1]


From another hand, due to [curtain limitations][2] in SharePoint Online, you can't create a new managed property with `Refinable` option enabled. 


  [1]: https://i.sstatic.net/dKQAR.png
  [2]: http://office.microsoft.com/en-us/office365-sharepoint-online-enterprise-help/manage-the-search-schema-in-sharepoint-online-HA103628856.aspx