I was recently assigned a task to customize the Search Core Results Web Part’s search results to include few additional columns for SharePoint 2010 online.
Came across the following helpful links
Here the first thing we need to do is to create a new Managed Property. After lot of struggle and searching came to know that this is not possible in case of SharePoint 2010 online.
SharePoint 2010 online administration center :-
However the good news was that creating managed property is now supported in case of SharePoint 2013 online.
Check out this article that explains all the differences in detail
Hope it helps!