Oscar has signed a new five-year deal with Chelsea.
The 23-year-old Brazil international joined the club from Internacional in the summer of 2012 and has been a vital member of the first-team since, scoring 11 goals last season.
The attacking midfielder has added four more goals in 15 appearances this term and has now committed his future to Stamford Bridge until 2019.
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"I am so happy because I love playing for Chelsea and living in England," he told the club’s official website.
"I have enjoyed playing here for two years, and now I have five more, so I am very happy."
Since Mourinho’s return to west London last summer Oscar has been earmarked for a pivotal role, with the Portuguese’s insistence that he play the number 10 role seen as a large factor in Juan Mata’s exit to Manchester United.
But Oscar has repaid his manager’s faith, and that of Roberto Di Matteo who signed him, with a total of 27 goals in 126 Blues appearances.
Oscar began his career in his homeland with Sao Paulo but soon moved to Internacional where he spent two years before moving to the Blues for a fee of just under £20million.
He scored twice on his first start for the club in Champions League and helped the team to win the Europa League in 2013.
Oscar has also amassed 40 caps for Brazil since making his debut against Argentina in 2011 and has scored 11 times, including the only goal in the 7-1 humbling by Germany in last summer’s World Cup semi-final.