The Mount Gambier product has played 97 games for Hawthorn since debuting in 2014, which included a career-high 19 games this year, but found himself on the outer at Waverley Park after the club stalled contract negotiations with the 27-year old.
According to reports from Fox Footy's Tom Morris, O'Brien spoke with Dogs head coach Luke Beveridge over Zoom and the utility is set to sign with the club on a multi-year deal and will play as a key defender alongside Alex Keath in 2022.
Port Adelaide were also interested in O'Brien to be used to support Charlie Dixon in attack, but will now turn their focus to signing Giants forward Jeremy Finlayson to replace tall Peter Ladhams, who has been told to explore other options.
Nothing has been set in stone, but it is understood that O'Brien will join the Kennel in the coming days.
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The Hawks' compensation for O'Brien's departure will not be a glamourous return as the key defender is an unrestricted free agent.
The 27-year old is known for his high-flying marks and will add a nice dynamic down back for the Dogs as they look to bounce back from their recent Grand Final defeat heading into the 2022 season.