The Nigerian is in Greece, where he completed his medical and signed on the dotted line.
The 24-year-old will be hoping to continue last season's form at Olympiacos, as he spent the past campaign on loan with Galatasaray.
Onyekuru said: "I know it is a big club in Greece, one of the biggest and they have good ambition and target every season and they are always in the Champions League.
"I also know of two Nigerians who played here before and I am just here to contribute and keep on pushing and also to be champions countless times, achieve more goals and aims with the club.
"I am happy to be here and ready to achieve more with the club."
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