O'Brien leaves Nottingham Forest for an undisclosed fee on a Wrexham contract that runs until the end of the 2027-28 season.
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"It's been a long time coming, but to be here is great," O'Brien said.
"The last couple of years has been a whirlwind for me and my family, but to sign a long-term contract and find somewhere I can call home is a great feeling.
"After speaking to everyone here, it's a club with a lot of ambition. I want to be part of that journey and take Wrexham to where they want to be."