The former Bhoys defender has joined the Scottish Premiership side.
After spells across the world, including in England, Ukraine and the USA, O'Dea is back in Scotland and has said he is happy to be 'back home,' according to BBC Sport.
Celtic fans haven't forgotten his time at Parkhead, though, and several sent a particular message to the defender on social media.
Indeed, plenty of Bhoys supporters replied to O'Dea's announcement on Twitter to wish him the best of luck. But some even told him they wished he was still with the Scottish champions.
Celtic have struggled defensively so far this season, with Dedryck Boyata, Jozo Simunovic, Tyler Blackett and more all underwhelming at various points in the campaign.
O'Dea, 28, might not necessarily be up to the required standard - having most recently spent time playing in India. Perhaps his Dundee spell could prepare him for a move back to the Hoops...
Well, that's what some supporters seem to want. Celtic fans reacted en masse with positivity as their former defender completed his Dundee move. Here's a selection of what they said...
@odea_darren best of luck Darren wouldn't have minded you back in the hoops HH— joesoap (@joesoap92031490) January 7, 2016
@odea_darren Could be doing with you back at Celtic mate— Mark Taylor™ (@MarkTaylor1988) January 8, 2016