It is absolutely not fair to compare Durant's and Oden's impact on their respect teams. Durant is the leader, playing on a team of inferior supporting casts. GO is a supporting cast playing on a team with superior players. Their roles are totally different, as well as their impact on their teams.
You do raise a worrying trend that Durant's team seems to play better without him, but I still think the sample size is much too small to judge.
Between GO and Durant, I don't think there's much doubt about who has been the better player.
Why is the sample size too small. The 82games.com win%, +/- and on/off is not based on games they won - but data specific to when the player is on the court even in games they lose. He played in 74 games this year and his win% from the time he is on the court is horrible. The net48 point differential when he is on the court is negative. He is bad from the +/-, win% and on/off statistics. What's so small about a sample of 74 games?
The problem is that they treat him like the leader of that team - but he gives them the on/off production of a backup.
For the issue of leadership - let's look at the on/off numbers. Brandon Roy is Portland's leader. Look at his on/off stats per 100 possession - Portland is up by 9.8 points per 100 possessions when Roy is on the court. He is clearly a leader of this team in his production.
Let's take a look at an efficient scorer on a horrible team - Kevin Martin of the Sacramento Kings - his on/off stat shows +3.3 points per 100 possession - the Kings are a better team when Kevin Martin is on the court - one of their problems this year was that he lost a lot of games and even in games he played - they could not play him long minutes.
Let's take a look Kevin Durant - an efficient scorer on a horrible team. We expect him to be at least as good as Kevin Martin, right? Well - no - actually the Thunder are up 8.9 points per 100 possessions when Durant is NOT on the court. Durant is not helping his team when he is on the court. He makes them worse. The best player on OKC - and the one that should be recognized as their leader is Russel Westbrook. The Thunder are 4.1 points per 100 possessions better when he is on the court than when he is not! The entire leadership argument is bogus. Kevin Durant does not make the Thunder better when he plays. He makes them worse. A whole lot worse.
Just for fun - let's look at Greg Oden's On/Off numbers - The Blazers are +2.4 points per 100 possessions when Greg Oden is on. The problem is that he is not on enough. Greg Oden is a more productive player than Pryzbilla - the problem Portland has with him - he can't stay out of foul trouble...
Just to give you an idea of how bad the situation is with Durant - if you look at his numbers from last year - they are the same - the Sonics were a better team when Durant was not on the court...
There is plenty of statistical evidence to show that this is a problem - the sample size is big enough... Kevin Durant is treated like a star by OKC - when he should not be given minutes until he improves his defense and team-play on offense.
The Blazers were very brave when they allowed Brandon Roy to be their leader at the tender age of 23 - but he is special as both a player and a person - he rose to the challenge. The Sonics were dumb beyond hope to trade Ray Allen for Jeff Green and throw the entire leadership thing on a 20 years old kid who is far from ready for it. He is not good at it with his production and he is too young to elevate the play of his teammates with him.
It is not all Durant's fault - the Blazers had continuity - they had a strong coach to lead the team, they even had a team leader on the court for Brandon's rookie year in Zach - which at least showed Roy what not to do. Durant did not have any of these things. He got a new coach in PJ who is not exactly known as leader of man, they shipped the veteran leadership that was there and they have thrown Durant onto the throne and told him "lead us". It failed miserably - and anyone that can not see it should really try to think what the goals of the game of basketball are...