Barcelona crushed Olympiacos with hat-tricks from Fermin Lopez and Marcus Rashford, who now has four goals from three Champions League games.
Lopez put Barcelona ahead with two deft first-half goals before a quick Olympiacos comeback was cut short by a Rashford double and a penalty from Lamine Yamal.
When goalie Konstantinos Tzolakis blocked the ball but was unable to hold it, Lopez was there to score past the defenders on the goal-line after Yamal had been played through on goal in the seventh minute.
The Spanish midfielder doubled Barcelona's lead half an hour later when he ran on to Dro Fernandez's ball into space, cut inside and curled a left-footed strike into the left corner.
The second half began in bizarre circumstances. Ayoub El Kaabi thought he had halved the deficit with a header, but after checking on the pitchside monitor, referee Urs Schnyder chalked off the goal for offside and instead awarded the visitors a penalty for a handball by Eric Garcia, which El Kaabi converted via the post.
Three minutes later Olympiacos midfielder Santiago Hezze was shown a second yellow card and ordered off for catching Lopez with a flailing arm.
This was a night when Yamal became the youngest player to reach 25 Champions League appearances – at 18 years and 100 days – and the teenager tucked away a penalty after Rashford was brought down while attempting to round Tzolakis.
Barcelona were not finished and were hungry for goals. In the 74th minute Rashford beat Tzolakis with a well-placed low finish, and then two minutes later Lopez smashed in his third from the edge of the box to become the first Spaniard to score a Champions League hat-trick for the Catalan side.
Rashford drilled his second after controlling a cross-field pass with a delightful touch. In 37 Champions League games for Manchester United, he scored more than one goal on just one occasion, but it has happened twice already with Barcelona.
Barcelona have not scored six goals in a Champions League game since their historic 6-1 victory over Paris St-Germain in 2017.
After losing to PSG by the same score and defeating Newcastle 2-1, Barcelona has six points after three games this season.
Olympiacos has now lost all 12 of its away Champions League games and is still on one point.
With an assist, Fernandez makes his European debut.
Barcelona head coach Hansi Flick surprised everyone by including 17-year-old Dro Fernandez. His calmness was demonstrated by a ball into Lopez's path for the game's second goal.
The latest talent off the famed La Masia production line, Fernandez made his first senior competitive start last month after taking part in Barcelona's pre-season tour of Japan.
Born in the small town of Nigran in the province of Galicia in north-west Spain, Fernandez joined Barcelona from Val Minor Nigran in 2022 and made an immediate impression playing with the under-15s.
An attacker who can play across the front line, Fernandez has Filipino heritage through his mother and has represented Spain at youth level.
He was among seven La Masia graduates named in the starting XI by Flick, illustrating the talent that has originated in the club's youth ranks but also the injury crisis faced by the first team.