The Swedish defender is focused on beating the Gunners, but admits the Greek side face a tough task to progress as they are relying on results elsewhere in Group F
The Piraeus side will book their ticket to the next round if they beat Arsenal on Tuesday night and Marseille fail to secure all three points against Borussia Dortmund in the other Group F encounter.
"We have to try to win against Arsenal and then we'll see what happens in Dortmund. Arsenal are already through but I don't know if that will be a big advantage," Mellberg told Uefa.com.
"Who knows, maybe [Arsene] Wenger will throw in all of the youngsters and they'll play an amazing game.
"We just don't know what they will do. But we are playing at home and we'll focus on our own game. Hopefully we can get the win."
Olympiakos were beaten 2-1 by Arsenal at the Emirates Stadium on matchday 2.