Hwang, 29, who has scored 15 goals in 45 international appearances for Korea Republic, signed for Forest from Bordeaux who were relegated from Ligue 1 at the end of last season. No fee was disclosed for the transfer.
"Nottingham Forest is delighted to confirm the signing of striker Hwang Ui-jo from French side Bordeaux," the newly-promoted English side said in a statement.
"He will immediately head out on loan to Greek champions Olympiakos for the remainder of the 2022/23 season."
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