Chelsea
On 16 July 2012, Chelsea agreed on a fee of R$61.575 million for Oscar. (according to several media, the fee was estimated for £19 million or £25 million) On 25 July, Chelsea announced the signing of Oscar on a five-year contract. On 19 August 2012, Oscar made his debut for Chelsea by coming on in the 64th minute for fellow debutant, Eden Hazard in their opening game of the 2012–13 season against Wigan Athletic.
Oscar made his first start of the season on 19 September in Chelsea's first Champions League match against Juventus. After making the match 1–0 in favour of Chelsea with a deflected effort, he added a second, which curled over Juventus keeper Gianluigi Buffon, to make it 2–0 at Stamford Bridge. However the game ended in a 2–2 draw, but he was awarded Man of the Match. He commented on his second goal, saying, "I am very happy with two goals on my debut but I would have been much happier if the team had won the game. In Brazil, young boys dream of playing in the Champions League and I'm very proud to have realised my dream."
Three days later, Oscar then made his first Premier League start in a 1–0 victory over Stoke City. Following his good performances, Oscar became a regular starter in Roberto Di Matteo's creative midfield triumvirate with Juan Mata and Eden Hazard. He scored his third goal of the season, in the Champions League, in Chelsea's 2–1 away defeat away to Shakhtar Donetsk in Donetsk, and continued his scoring record in Europe by scoring into an open net from 40 yards out in the reverse fixture against Shakhtar at Stamford Bridge, a match which Chelsea won 3–2. Oscar also scored in Europe in ultimately Chelsea's last Champions League fixture, making the score 6–1 for Chelsea against debutants Nordsjælland.
After the sacking of manager Di Matteo on 21 November, interim manager Rafael Benítez only started Oscar three times in his first seven games. Benítez praised Oscar's attitude after dropping him for the FIFA Club World Cup Final against Corinthians and believed there was no rift between himself and the player. He scored his first Premier League goal for the club on 23 December, scoring a penalty during an 8–0 win against Aston Villa.
On 14 February 2013, less than a minute after coming off the bench to replace Mata, Oscar exchanged passes with Hazard before finishing past Tomáš Vaclík, breaking the deadlock and handing Chelsea a 1–0 away victory over Sparta Praha in the first leg of their Europa League Round of 32 clash. On 15 May, Oscar played the full ninety minutes as Chelsea defeated Benfica 2–1 in the Europa League final in Amsterdam, securing his first piece of silverware with the London club. The following day, his goal against Juventus in the Champions League was named the Chelsea "Goal of the Season".
In the first match of the 2013–14 season on 18 August, Oscar scored Chelsea's opening goal in a 2–0 win over Hull City. He opened the scoring against local rivals Fulham in a 2–1 win on 21 September. On 1 January 2014 Oscar put in a match-winning performance against Southampton at St Mary's Stadium, scoring one and assisting two as Chelsea won the match 0–3. However, he was booked in that match for diving. Oscar scored fourth and fifth goals in Chelsea's 6–0 victory over Arsenal on 23 March, which was Arsène Wenger's 1000th match in charge of the club.
Before the start of the 2014–15 season, Oscar's shirt number 11 was given back to the returning Didier Drogba, and he instead took the number 8 vacated by Frank Lampard earlier in the summer. He said "Didier is a Chelsea legend and a senior player. I am happy for him to take the 11 and for me to take the 8. Lamps is another legend at this club and I hope I will be as successful in the shirt as he was." His early season success was rewarded in November as he signed a new contract with the club keeping him with the Blues until the end of the 2018–19 season. Oscar scored twice in the first half of Chelsea's 5–0 away win at Swansea City on 17 January 2015, exploiting a wayward pass by Gylfi Sigurðsson to open the scoring after 50 seconds. On 26 April, he was hospitalised after suffering a head injury following a collision with opposing goalkeeper David Ospina in a 0–0 away draw against Arsenal in the Premier League; he was released the following day.
Oscar scored Chelsea's first goal of the next season in an eventual 2–2 home draw with Swansea, being substituted early in the second half after the dismissal of goalkeeper Thibaut Courtois. On 16 September, he scored a penalty in a 4–0 win over Maccabi Tel Aviv in the first game of the Champions League group stage. He, Diego Costa and Cesc Fàbregas were abused by supporters who believed that the trio's slump in form had cost the job of popular manager José Mourinho; in the first game since his dismissal, on 26 December, Oscar slipped and missed a spot kick in a 2–2 home draw against Watford. The following 31 January, Oscar scored a first-half hat-trick in a 5–1 win at Milton Keynes Dons in the fourth round of the FA Cup.
On 26 October 2016, Oscar made his 200th appearance for Chelsea in a 2–1 League Cup defeat to West Ham United.