The 45-year-old Italian dismissed the Black Cats as "weak" as a club and some of its players as "cowards", and was particularly critical of skipper John O'Shea, Phil Bardsley and Lee Cattermole.
In his comments over the weekend, Di Canio suggested he was too good for Sunderland and confirmed his intention to make his name as a manager in England with another Premier League club.
It is not the first time he has found himself in the firing line with O'Neill having dismissed him as a "managerial charlatan" following his appointment as Republic of Ireland manager in the wake of criticism of his own time with the Black Cats.
The Wearsiders hit back in a statement on Monday afternoon.
SAFC statement: "Sunderland AFC would like to express its disappointment in relation to the disparaging comments made recently by Paolo Di Canio regarding the club and its players.
"The club is immensely proud of its players for the dignified and restrained manner in which they have conducted themselves publicly since Mr Di Canio's departure, and it is particularly disappointing to read such comments when there are legal obligations in place to ensure such behaviour does not occur.
"The club is now considering its position with its legal representatives.
"Having reached a cup final for the first time in 22 years, whilst also enjoying a run of only one loss in 12 games, we want to focus on what is a positive time for the football club and we would hope it is possible to draw a line under this matter quickly and that there will be no repetition.
"Neither the club, head coach Gus Poyet nor the players will be making any further comment on the situation.
"We are looking forward, not back and are focusing on the vital games we have ahead of us."
Tuesday 28th January 2014
Man Utd V Cardiff
Norwich V Newcastle
Southampton V Arsenal
Swansea V Fulham
Crystal Palace V Hull
Liverpool V Everton
Wednesday 29th January 2014
Aston Villa V West Brom
Chelsea V West Ham
Sunderland V Stoke
Tottenham V Man City
SUNDERLAND 1 KIDDERMINSTER 0
32. Oscar Ustari
12. Ondrej Celustka
29. Valentin Roberge
21. Modibo Diakite
27. Santiago Vergini
22. El-Hadji Ba 66'
33. Lee Cattermole 72'
7. Sebastian Larsson
23. Emanuele Giaccherini 66'
35. Charis Mavrias
17. Jozy Altidore
Subs
41. Duncan Watmore
4. Ki Sung-Yueng
8. Craig Gardner 66'
13. Jordan Pickford
14. Jack Colback 72'
16. John O'Shea
31. Fabio Borini 66'
Charis Mavrias' first Sunderland goal was enough to see off Kidderminster 1-0 and fire the Capital One Cup finalists into the FA Cup fifth round.
The Greece international's fifth-minute strike at the Stadium of Light ended non-league Harriers' dreams of reaching the last 16 for the second time in their history.
But it was far from a classic as Gus Poyet's much-changed team laboured to beat a club who started the game 79 places below them in the league pyramid three days after denying Manchester United a League Cup final trip to Wembley.
Indeed, had striker Michael Gash taken the glorious first-half opportunity which came his way, the Skrill Premier outfit might even have booked themselves a replay.
However, he did not and the Black Cats, who have won all seven of the cup ties they have played at home this season, have now lost only once in all competitions in 12 attempts.
Poyet had expressed his intention to use the game to run the rule over some of his fringe players, but what he saw can only have confirmed misgivings about some of them.
The Uruguayan vowed not to make 11 changes from the side which performed so heroically at Old Trafford in midweek, and he was as good as his word, if only just.
Only Lee Cattermole was retained as Poyet tested the depth of his squad, and then meant debuts for January signings Oscar Ustari and Santiago Vergini and a first start for midfielder El-Hadji Ba.
The Black Cats were in front with just five minutes gone when Harriers defender Josh Gowling fired his attempted clearance straight at Mavrias, who gratefully accepted the opportunity to open his account at the Stadium of Light.
But as the home fans settled down for what they hoped would be a regulation victory, Kidderminster very nearly dragged themselves back into the game within five minutes.
Emanuele Giaccherini and Cattermole combined to serve the ball up to Gash, who ran in on goal before unleashing a right-foot shot which very nearly squirmed through keeper Ustari's dive.
Gash, playing as a loan frontman, battled gamely to put Vergini and central defensive partner Modibo Diakite under pressure, but Ustari was otherwise untroubled with Sunderland dominating possession.
They prospered in particular out wide, where full-backs Ondrej Celustka and Valentin Roberge linked well with midfielders Sebastian Larsson and Mavrias to pin Kidderminster back.
However, they created few chances of note as a rather drab first-half limped towards its conclusion.
Cattermole forced a rare save from keeper Danny Lewis from 30 yards and Giaccherini shot wastefully wide from a promising position inside the penalty area before blazing a later effort high over.
Roberge saw another attempt from distance drop on to the roof of the net with the aid of a deflection as Lewis found himself largely untroubled.
The keeper might have been in serious trouble within two minutes of the restart when the ball dropped to Ba on the edge of the box, but the Frenchman's scuffed shot trickled into his arms.
However, with captain Kyle Storer and Jack Dyer starting to get a grip in central midfield, the visitors began to string passes together to lay a foundation for something more progressive.
Lewis was troubled on the hour when Vergini unleashed a 30-yard effort which he could not hold, although the element of surprise as the ferocity of the shot was perhaps partly responsible for his discomfort.
Ba floated a curling 64th-minute attempt over the angle of bar and post as Sunderland attempted to restore a measure of control, and Poyet introduced Craig Gardner and Fabio Borini in place of the ineffective Giaccherini and Ba in an attempt to aid that process.
In the meantime, Storer had been fortunate to pick up only a yellow card for an ugly challenge on Roberge.
Borini saw a close-range shot charged down with the home side looking belatedly to kill the tie off, but it was all a little limp as the game crawled towards its conclusion with little urgency.
However, Ustari found himself in the firing line twice within seconds as full-back Danny Jackman drilled wide and then substitute Freddie Ladapo blasted an 88th-minute thunderbolt inches past the far post.
But it took a remarkable injury-time block by right-back Lee Vaughan to prevent Borini from extending the lead in a finish which had little in common with much of the rest of the game.