Board Thread:Arrow discussions/@comment-807326-20170330004818/@comment-16581186-20170330124734

They met in the Flash part of the Invasion crossover.

And Ollie barely knowing Supergirl makes it even more convenient for him to call her. He doesn't want to put anyone he cares about in danger. And he knows enough considering he's seen what she can do and that she's basically indestructable. If he wanted a Supergirl contact, Cisco would have given him one.