Heartbreaking... but life is sometimes like that.
To me Ollie was killed and Tony could't accept that. He was clutching for straws, maybe because it was the only way he could go on.
Personally, I can relate - or at least I feel like it - to Tony. If I had lost a child like that, it would be very difficult for me to accept that.
To have made actually such a small mistake, but it had such harsh consequences.
Of course I am not happy with the ending, it was merciless to Tony, but I can understand the writers. Tony's search was probably unjustified from the very beginning of the events, so he just continued that way. We might think he lost it in Russia/Romania or whereever, but he had lost it already because the only thing that kept him alive was the search for Ollie.