A combative midfielder, O'Connor was signed from Port Vale in the summer of 2016 and appointed club captain by then-manager John Sheridan.
The Northern Irishman, capped 11 times by his country, won promotion in successive seasons with Rotherham United and has experience of playing at a higher level from his time with Scunthorpe United.
After two years at Port Vale, the central midfielder turned down the offer of a new contract with the Valiants in favour of a move to Meadow Lane.
Home kit: Available
Away kit: Available
Third kit: Available
Boots & socks: Ted Humphrey