The 22-year-old, who had a spell on loan with Burton during the 2012/13 season, turned down the offer of a new deal at Plymouth after his contract expired in June.
O’Connor told Burton’s official website: “It feels great to be back. Obviously this is the place where I spent my first season in the Football League.
“It was under a different manager then but I’m really excited about working under the new gaffer here.
“When I played against Burton last season for Plymouth, I could really see how much the club had come on.”
Irishman O’Connor started out at Blackburn and after loan spells at Burton, Torquay and Plymouth made a permanent move to the latter last summer.
He made a total of 48 appearances for Plymouth, scoring three goals.