Campbell, on-loan from Arsenal, struck in last night's 2-0 win against United.
He told the Daily Mal: "They showed an interest in me when I was moving to Europe but I have been an Arsenal fan since I was a child and that's why I chose them.
"This [Olympiakos] has been a very good experience.
"I am trying to do my best, to do better and continue growing as a footballer. It is why I chose Olympiakos.
"This is a big team with the opportunity to play in the Champions League, the most important competition in Europe."
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