The Reds, beaten 2-0 by Cox's former club West Bromwich Albion on Friday, had concentrated previously on strengthening their defensive options but a new forward was always part of Sean O'Driscoll's thinking with the new season now just days away.
25-year-old Ireland international Cox, who has tweeted his delight at securing a move to the City Ground, joined West Brom for £1.5m in the summer of 2009.
He scored 10 goals in the following season as the Baggies finished second in the Championship, one place and twelve points ahead of Billy Davies's Forest side.
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