Click here for a complete list of the Oscar winners 2016
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The actor was the favourite to win this year for his role in 'The Revenant', but it was still a stunned and emotional Leo who took to the stage to accept his Award from last year's Best Actress winner, Julianne Moore.
Leo told the audience, who gave him a standing ovation:
Thankyou all so very much. Thankyou to the Academy, to all of you in this room, to the other nominees for their incredible performances, the cast and crew, my brother in this endeavour Tom Hardy... to Mr Alejandro Inarritu, what an unbelievable talent you are...
He included many names, including one of his favourite directors Martin Scorsese. Then he joined in the political theme of the night, lending air to his own passion of climate change.
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He said: "Making 'The Revenant' was about man's relationship to the natural world in 2015, the hottest year in history.
"Climate change is real, it is happening right now. It is the most urgent threat facing our entire species and we need to stop procrastinating, we need to support leaders around the world who speak for all of humanity, for the billions and billions of underprivileged people, for our children's children, and for those whose voices who have been drowned out by the politics of greed. Let us not take this planet for granted. I do not take this award for granted."