The Brazilian is now with Shanghai SIPG having left the Blues in December 2016.
He worked under Mourinho at Stamford Bridge and admits it wasn't always easy.
Oscar told ESPN Brasil: “Sometimes it seems difficult to work with him, but he does it because he demands the most, he takes you out of the comfort zone, he's very good indeed.
“I had a lot of good times with him, he was a guy who helped me grow a lot in football, he trusted me and praised me a lot to the press, I really enjoyed working with him."
He added: “Mourinho likes to work the psychological side of the athlete.
“He always wants the player to be happy to play.
“When you think you will not play, you play. When you think you're good, he spares you a game. He likes to leave the player really well focused in the game and wanting to show the best football. He has this way to do this."