Neither side would give an inch in the ensuing shootout, with both goalkeepers also scoring, but O'Connell fluffed his spot kick to see Forest prevail 10-9 at the City Ground.
Fleetwood Town, Hull City, Cheltenham Town, Millwall, and Wycombe Wanderers also needed penalties to progress to the second round, with Crewe Alexandra, Sheffield United, Colchester United, Gillingham and Northampton Town suffering heartbreak.
The other teams to go into the hat for the next round are Blackpool, Blackburn Rovers, Rochdale, Leeds United, Derby County, Lincoln City, Preston North End, Rotherham United, Doncaster Rovers, Burton Albion, Walsall, Bristol Rovers, Newport County, Norwich City, AFC Wimbledon, Brentford and Forest Green Rovers.
Results in full:
Nott'm Forest 1-1 Bury (10-9 on penalties), Blackpool 3-1 Barnsley, Bristol City 0-1 Plymouth Argyle, Bristol Rovers 2-1 Crawley Town, Cambridge United 1-4 Newport County, Carlisle United 1-5 Blackburn Rovers, Cheltenham Town 2-2 Colchester United (6-5 on pens), Crewe Alexandra 1-1 Fleetwood Town (3-4 on pens), Exeter City 1-1 Ipswich Town (4-2 on pens), Grimsby Town 0-2 Rochdale, Leeds United 2-1 Bolton Wanderers, Macclesfield Town 1-1 Bradford City (4-2 on pens), Mansfield Town 6-1 Accrington Stanley, MK Dons 3-0 Charlton Athletic, Middlesbrough 3-3 Notts County (4-3 on pens), Millwall 0-0 Gillingham (3-1 on pens), Norwich City 3-1 Stevenage, Oldham Athletic 0-2 Derby County, Oxford United 2-0 Coventry City, Portsmouth 1-2 AFC Wimbledon, Port Vale 0-4 Lincoln City, Preston North End 3-1 Morecambe, Queens Park Rangers 2-0 Peterborough United, Rotherham United 3-1 Wigan Athletic, Scunthorpe United 1-2 Doncaster Rovers, Sheffield United 1-1 Hull City (4-5 on penalties), Shrewsbury Town 1-2 Burton Albion, Southend United 2-4 Brentford, Swindon Town 0-1 Forest Green Rovers, Tranmere Rovers 1-3 Walsall, Wycombe Wanderers 1-1 Northampton Town (7-6 on pens), Yeovil Town 0-1 Aston Villa, Reading 2-0 Birmingham City, West Bromwich Albion 1-0 Luton Town