Teenage star Lamine Yamal was also on target for Hansi Flick's side, who lost against holders Paris Saint-Germain in their previous European outing in the league phase.

After a Lopez double, Ayoub El Kaabi pulled one back from the penalty spot for the Greek champions, but Santiago Hezze's red card allowed injury-hit Barcelona to rack up a big scoreline and go provisionally third.

Flick was without Robert Lewandowski, Raphinha and several other regulars, but his team were able to build some confidence at the Olympic stadium ahead of the Clasico against Real Madrid on Sunday in La Liga.

"I'm very happy, we needed a win like that, we ended it feeling good and now to think about the Clasico," Lopez told Movistar after his treble.

"I'm grateful to the team and want to continue in this dynamic."

Barcelona only just scraped a win over Girona on Saturday in a tight Catalan derby, and almost fell behind right at the start against the Greeks.

Alejandro Balde gave the ball away and veteran goalkeeper Wojciech Szczesny had to get down well to deny Daniel Podence.

Balde quickly made amends in attack as his driving runs helped Barca carve open the visitors.

Rashford volleyed narrowly over after 17-year-old playmaker Dro Fernandez and Balde connected on the left.

Balde was involved again as Barca took an early lead, feeding Lopez, who played in Yamal.

Olympiacos goalkeeper Konstantinos Tzolakis got down well to dispossess the teenage winger, but Lopez was on hand to bundle home the loose ball.

Rashford drilled a free-kick into the side-netting as Barca controlled the game, although they lacked focus in defence, which has been a consistent problem this season.

With Sunday's Clasico beckoning, their lapses in concentration are a worry.

- Olympiacos lifeline short-lived -

Lopez netted his second after 39 minutes, having been played in by Dro on his Champions League debut.

The midfielder almost scored his hat-trick early in the second half but Tzolakis tipped his header around the post from Yamal's cross.

Olympiacos pulled their way back into the game through Moroccan striker El Kaabi.

The forward got between Pau Cubarsi and Eric Garcia to head home, and although the goal was disallowed for offside, Olympiacos were then awarded a penalty for handball by Garcia.

El Kaabi slotted past Szczesny to give his team a lifeline, but minutes later their chances faded.

Barca's Marc Casado slumped to the ground after a seemingly innocuous contact with Hezze, who was harshly shown a second yellow card.

Rashford won a penalty after tumbling under Tzolakis's challenge, and Yamal sent the goalkeeper the wrong way to extend Barca's lead after 68 minutes.

In fine form during his loan from Manchester United, Rashford netted two goals in six minutes, either side of Lopez's third as the Catalans ran amok.

Rashford twice beat Tzolakis with clinical low near-post finishes to reach four goals in the Champions League this season after his brace in their win at Newcastle.