Lee, bidding for Malaysia's first Olympic gold medal after winning silver to Lin Dan at Beijing and London, laboured early against Singaporean Derek Wong but stepped up a gear to close out the pool match 21-18 21-8 at the Riocentro.
"I am okay. I think in the first game I made a lot of simple mistakes and in the second game I increased my speed and made less mistakes," the 33-year-old told reporters.
Lee is under huge pressure to bow out with a title in his Olympic swansong, having been ranked the world number one for much of his career but never won a major trophy.
"Everyone is hoping for a gold medal from the Olympics," he said. "I am not able to predict the result. As this is my last Olympics, it is more important for me to just fight, fight and fight."
Home favourite De Oliveira, born and raised in a favela north of the Olympic Park, enjoyed another round of fervent support from local fans as he bowed out swinging in a 21-12 21-12 loss to German Marc Zwiebler.
Although it was the 19-year-old's second loss following Saturday's opening defeat to Irishman Scott Evans, he paraded shirtless around the courts as if having won the gold, bashing shuttle-cocks into the roaring crowd.
De Oliveira's flamboyant farewell followed a more sombre exit for India's first badminton medallist Saina Nehwal, who limped out of the arena after an injury-hampered 21-18 21-19 loss to Ukrainian Maria Ulitina.
The fifth seed, who became a huge celebrity in her home nation after her bronze-winning effort at London, said she had injured her right knee about two weeks before the Games and it had failed to recover in time.
China's third seed Lin, bidding to become the first player to win titles at three different Olympics, moved safely into the last 16 with a 21-7 21-12 win over Vietnam's Nguyen Tien Minh.
Lin's compatriot and women's singles champion Li Xuerui also marched on, defeating the United States' Iris Wang 21-16 21-12.
(Editing by Greg Stutchbury)
