Having come through the youth ranks at Huddersfield, O’Brien had become a mainstay of the Terriers’ side over the past few seasons.
The 23-year-old made 131 senior appearances in all competitions for the Terriers, scoring eight goals in that time, and helping the club to last season’s Championship play-off final.
But after missing out on promotion following defeat to Forest themselves at Wembley, the midfielder has now completed a move to The City Ground on a long-term contract.
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That of course means he will be playing top-flight football next season, and it seems Cooper is expecting his latest signing to make a big impact in the coming campaign.
Given his reaction to the signing of O’Brien, the Forest boss told his club’s official website: “We are delighted to secure the signing of Lewis O’Brien and we are thrilled to get this one over the line.
“He was one of the standout players in the Championship last season and possesses many of the qualities that we are looking for to strengthen the midfield unit.
“At 23 years of age and with an abundance of experience already, I am confident Lewis will play a pivotal role next year and I look forward to watching him grow as a player over the coming months.”
O’Brien becomes the second Huddersfield player to complete a move to Forest during the current market, with left-back Harry Toffolo having also joined the newly promoted Premier League club on Friday.
Just as was the case with Toffolo, the departure of O’Brien will be a big blow for Huddersfield.
The midfielder was a key figure in the centre of the park in the past few seasons, proving himself an important driving force for the Terriers.
As a result, there will be plenty at the club who are no doubt sad to see him go, not least given he is a player who had come through the youth ranks with the club.
However, this is certainly an exciting signing for Forest, and based on what Cooper has said here, it will be interesting to see just how much he is used, and how he fares next season, and beyond.