Phil Parkinson declared his business ‘sensible’ as his side tries to force their way up the table in what remains a remarkably open promotion in battle in League One.
You can read our analysis of their deals here.
But what business did Parkinson’s rivals do and which deals look they like they could be crucial in the final months of the season?
Nathan Broadhead explains why he opted to join Wigan Athletic after Sunderland interest
Alex Neil explains what happened with Nathan Broadhead, Sunderland, Wigan Athletic and Everton in transfer verdict
'Bottle job move' - Sunderland fans offer opinions on Nathan Broadhead's move to Wigan Athletic
Sunderland AFC transfer gossip: Cats agree to sign transfer listed ex-Wigan Athletic defender - reports
Transfer news: Leeds United sign Sunderland 'target' with ex-Wigan Athletic and Everton making Premier League switch
We take a closer look...
IN: Curtis Tilt, Hakeeb Adelakun, Josh Koroma
Rotherham held on to their best players and the unit that is beginning to build serious momentum in the promotion push.
To that group they have added proven quality from the Championship and one of the most highly-rated League One defenders of recent times in Curtis Tilt.
Adelakun and Koroma bring the skill and delivery so crucial to Warne’s system, one that has overpowered an eye-catching number of opponents in the top half of the table since the turn of the year.
They are now automatic promotion contenders beyond any doubt.
IN: Jason McCarthy, David Stockdale
OUT: Rolando Aarons (Loan expired)
Gareth Ainsworth admitted that a couple of attempted deadline day deals did not come off, and the departure of Rolando Aarons means that they are looking just a touch light in the forward areas.
It is of course all about perspective for Ainsworth and his club, who have had a remarkable season so far and are expected to enjoy a major financial boost in the near future when a takeover is completed.
As it stands, however, their dip in form means it is tough to see them keeping their place at the top.
IN: Jack Taylor, Reece Brown, Sammie Szmodics
OUT: Alex Woodyard, Marcus Maddison
The prospect of losing key players was a major concern for a side whose form had rapidly declined towards the end of the year.
That they have begun the month with their hope reignited is an impressive feat.
Marcus Maddision left on loan as his contract winds down, but good planning means it looks as if they will not be overly affected.
Sammie Szmodics, on loan from Bristol City, is an exceptional replacement and is already making a major impact.
The key was always going to be hold onto Ivan Toney, the best player outside the top-two tiers.
That they have done so keeps their promotion ambitions alive.
IN: Josh Earl, Levi Andoh
OUT: Aristote Nsiala
Paul Lambert had been candid throughout the window, stating clearly that he would not have the funds to invest heavily in his squad.
Josh Earl looks a smart addition to a squad with plenty of depth.
They do not look anything like the force they were in early season but they have so many options a turn in form seems inevitable.
The key was not losing the talented young players key to their strong start, such as defender Luke Woolfenden and midfielder Flynn Downes.
IN: Ryan Giles
OUT: Sam McCallum
Mark Robins had not expected to do any January business but managed an impressive late capture.
Ryan Giles had impressed at Shrewsbury in the first half of the season, and the Wolves youngster will bring yet more pace to a team in excellent form and full of energy.
They managed to avoid any of their young talent in a strong midfield department, and though Sam McCallum was picked up Premier League side Norwich City, he has been loaned straight back and so there is little disruption to a team that might have felt vulnerable at the start of the month.
IN: Reeco Hackett-Fairchild, Steve Seddon, Cameron McGeehan
OUT: Anton Walkes, Luke McGee
Portsmouth’s form had started heading in the right direction before the window opened and they have gone from strength to strength since.
Their additions look very canny.
Cameron McGeehan is not only a proven promotion winner at this level, but looks a natural fit for Kenny Jackett’s side and has quickly settled into his midfield.
Steve Seddon is already proving to be an inspired addition, contributing significantly in the final third since his arrival.
IN: George Thorne, Liam Kelly, Nathan Holland, Marcus Browne
OUT: Jordan Archer, Shandon Baptiste, Tariqe Fosu
Midway through the window it looked as if Oxford were going to have a window the envy of the rest of the division.
They had moved quickly to bring in some exciting young talent and looked to be taking a real wealth of attacking options into the latter stages of the season.
The picture changed dramatically on the eve of deadline day, when it became clear that Shandon Baptiste and Tariqe Fosu would be moving to Brentford.
Karl Robinson had publicly raised the stakes earlier that day by making clear his intention to bring in Will Grigg and a right-back.
He ended up with neither.
Still, that early business means they can still push for the top positions and they underlined that with a good win over Blackpool just hours after the angst of losing those two players so key to their impressive early-season performances.
IN: Callum Connolly, Lewis Gibson, Barrie McKay, Lewie Coyle, Glenn Whelan, Isak Snaer-Thorvaldsson
OUT: Jimmy Dunne, Ash Hunter, Peter Clarke, Ash Eastham, Conor McAleny
Joey Barton made the bold call of letting Ash Hunter and Jimmy Dunne go, stating issues behind the scenes meant their position in the squad was untenable.
When Peter Clarke joined Tranmere Rovers, their squad suddenly looked very light.
They have done some savvy business since then, bringing in energy and quality to balance the picture somewhat.
IN: Devante Cole, Fejiri Okenabirhie, Jacob Ramsey, Jason Lokilo
OUT: Alfie May, Kwane Thomas
Doncaster have been in excellent defensive form of late, conceding just two goals in the month of January.
Where they have perhaps been a touch lacking is up front, and to that end, late swoops for Cole and Okenabirhie will give Darren Moore and supporters some hope that they can push on in the latter stages of the season.
It remains remarkable that they have not had to fight off stronger interest in the excellent Ben Whiteman.
IN: Joe Powell, Conor Shaughnessy, Jamie Murphy
OUT: Liam Boyce, David Templeton
Burton went into the window with Nigel Clough typically honest in his assessment of where his club stood.
Star players Liam Boyce and Scott Fraser were entering the last six months of their deal and Clough admitted he could lose both.
Boyce did indeed move to join Hearts, but Fraser stayed and Clough used his connections to ensure he was very well replaced.
Jamie Murphy has already made a big impression since joining from Rangers, having thrived under Clough’s management earlier in his career.
Powell looks a clever addition in midfield and holding onto Fraser is key. There is no midfielder with a better output in the final third in the division this season.
*Only players who came into the window as part of a club’s first-team group have been included in outgoing business