Oscar signed for the club when Roberto Di Matteo was in charge and his importance for the side has grown ever since, with his talent really showcased when he scored a stunning brace in a 2-2 draw with Juventus in the Champions League in his debut season.
He told the Chelsea FC website: He said: "I am so happy because I love playing for Chelsea and living in England. I have enjoyed playing here for two years, and now I have five more, so I am very happy."
The Brazilian has so far made 126 appearances for the club, scoring 27 goals since his arrival from Internacional.
He has played a big part in helping Chelsea to the top of the Premier League, with the side only dropping four points during the campaign, currently top of their Champions League group, and in the quarter-finals of the League Cup.
At 23, Oscar has his best years ahead of him, and the deal ties down the Brazilian for his best years ahead, or at least secures a big fee should anybody come calling, but given how often he has featured for Jose Mourinho's side this season, it would be a hard task for any club to prise him away from Stamford Bridge.
The talented Brazilian seems to be a very important part of Mourinho's side. Not only is he a gifted payer with the ball at his feet, and a good football brain to unlock defences and score goals, but his work rate and ability to help out his defenders does wonders for his side - and it is of little wonder that Jose Mourinho is a fan of his.
He has also represented his country on 48 occasions, scoring 11 goals, and looks as though he will be vital to his country's fortunes in the near future.