The 21-year-old has impressed during the Blues' unbeaten start to the season, scoring in the opening day win against Hull City and collecting the Man of the Match award in the process.
Oscar, who joined from Internacional last summer, has already worked under three different bosses since arriving at Stamford Bridge, but the Brazilian hailed the impact of Mourinho's return.
"It's been great to work with Jose Mourinho. He is one of the best coaches in the world," he told reporters.
"We have seen he has already had a lot of success at Chelsea and I am just really happy to have the opportunity to work with him.
"You can already see the influence he has had on us during the first few games and I hope the team will be able to continue to grow under him, as I will also be able to grow as a player.
"As early as pre-season we could see the belief with which he works."
Chelsea have enjoyed a strong start to the campaign, collecting seven points from a possible nine, something which Oscar believes Mourinho deserves credit for.
"We could see the confidence he has with us as players but also the confidence in his own work. It is clear to us that he is a great coach," he said.
"I feel a lot more confident this year and I am sure that I am going to score a lot more goals. I am ambitious for this season and I am happy with the way things are going for me at the moment.
"We have started well and I want to make sure that this is a big year, a year in which Chelsea win titles."