The winner of the OGAE poll and the actual winner of Eurovision were the same in only four out of eight years. Sweden, Norway and Denmark were huge favorites long before the contest but I'm not sure about Serbia. In their poll, Germany was only in third place and in the other three years, including last year, the actual winner wasn't even in their top 3. This year looks a lot like 2011, so it's likely that their ranking will be quite different again.
I think, whoever will win in their poll, will not be the winner on May 23. Winning the OGAE poll this year should be a reason for the bookies to downgrade the entry.