Manager Arsene Wenger was left to rue his side's missed chances after Arsenal threw away a two-goal lead to allow Schalke 04 to fight back and draw 2-2, leaving group B wide open.
The draw leaves Schalke top of the table with eight points while Arsenal are just a point behind in second.
Olympiakos are third on six points after their 3-1 win over bottom-placed Montpellier, leaving all still to play for in the group.
"It's an open group. I believe we are in a good position where you want to win your home game," said Wenger with Arsenal hosting Montpellier, bottom of group B, in a fortnight while Olympiakos are at Schalke the same night.
"Schalke have to win their game against Olympiakos as well, so it will probably come down to the last game."
England forward Theo Walcott squandered two clear chances to win the game, hitting the post in the final 15 minutes, then firing straight at Schalke goalkeeper Lars Unnerstall with virtually the last kick of the game.
"It's regrettable that we conceded a goal before the break, then missed a goal with the last chance of the game. They both had a great influence on the result," admitted Wenger. Walcott proved his recent injury is behind him on 18 minutes with the opening goal before France's Olivier Giroud added the second on 26 minutes with a header from a Lukas Podolski cross.
