Key Points

Martinelli reacted quickest in the 12th minute after Viktor Gyokeres' shot was saved by Konstantinos Tzolakis and deflected onto the post. The Brazilian forward also had a goal and an assist in Arsenal's opening win over Athletic Club.

Arsenal manager Mikel Arteta rotated heavily, making six changes from the weekend victory over Newcastle.

Olympiacos threatened an equaliser as Daniel Podence forced David Raya into a strong save and Francisco Chiquinho had a goal disallowed for offside.

Martin Odegaard was denied by Tzolakis before substitute Bukayo Saka sealed the win in stoppage time, his low shot slipping through the goalkeeper's legs.

"Winning the Champions League is always very complicated, keeping another clean sheet, we make something like 11 in 14 games, which is remarkable from the boys. I think we started the game really well, I think we looked a real threat, really dynamic, really playing forwards around a lot of threats. Generally to the goal another two or three big chances and we don't put them away in this competition," Arsenal manager Mikel Arteta said.

Gunners will be back at Emirates Stadium on Saturday to take on West Ham United in the Premier League. That's the last game before the October international break, they will resume away to Fulham on October 18.

Elsewhere, Anthony Gordon's pair of coolly-taken penalties helped Newcastle United to their biggest-ever win in the Champions League proper by beating Union Saint-Gilloise 4-0.

Summer signing Nick Woltemade gave Eddie Howe's side a 17th-minute lead when he diverted Sandro Tonali's low shot past a wrong-footed Kjell Scherpen.

Gordon struck twice from the spot either side of half-time and substitute Harvey Barnes capped victory with just over ten minutes remaining by finishing off a counterattack.

- IANS