Barnard was previously on loan at League One side Bournemouth, where he scored four times in 15 games earlier this season.
The 28-year-old is Oldham's third capture of transfer deadline day, following Reading's teenage midfield player Jordan Obita and veteran striker Chris Iwelumo into the club.
The signings come as Paul Dickov bids to guide the 19th placed Latics away from the danger, sitting just one point above the relegation zone.
The 25-year-old was one of the mainstays of Southampton's promotion campaign last season, but he has found first-team opportunites limited since the Saints entered the top flight.
The Belgian has been restricted to just seven appearances in all competitions this season, although he has featured in four games since the turn of the year.
De Ridder, who spent three years with Dutch side De Graafschap before moving to Southampton in the summer of 2011, becomes Dougie Freedman's fifth signing of the transfer window.
Jay Rodriguez gave the Saints a shock third-minute lead when he latched onto a poor back pass from Michael Carrick to tuck home around the advancing David De Gea.
But Rooney levelled just five minutes later, slotting into the bottom corner following an excellent through ball from Shinji Kagawa.
Kagawa then hit a post on United's next attack and it took some excellent goalkeeping from Artur Boruc to keep the hosts at bay as returning Robin van Persie and Danny Welbeck both threatened danger.
United finally doubled their advantage on 27 minutes when Patrice Evra climbed highest at the far post to meet Van Persie's free-kick, the France defender leaving Rooney with the easiest of tap-ins from barely a yard.
But the hosts were then forced to survive a second-half Southampton bombardment, with the visitors spending almost the entire second period camped in the United half.
Mauricio Pochettino's team pushed forward in search of a leveller, with substitute Adam Lallana having a significant influence as Rickie Lambert twice went close.
Briefly, United relieved the pressure, with Van Persie's goalbound header brilliantly saved by Boruc before the Dutchman had an effort ruled out from a Rooney cross for offside.
Southampton were soon back on the offensive, though, with De Gea producing an excellent flying save to turn away Lambert's free-kick.
Nemanja Vidic flung himself in the way of another Lambert piledriver before Rooney, who finished playing on the left as defensive cover for an overrun Evra, ensured the Saints' final assault got nowhere and his side kept the valuable three points.
The visitors took a shock third minute lead via Jay Rodriguez after a poor back pass from Michael Carrick.
Wayne Rooney put United well in control with a quick-fire brace before the Saints dominated the second half but could not find the equaliser their play deserved.
The defeat leaves the Saints hovering three points above the drop zone and Pochettino told Sky Sports: "I'm very frustrated and angry because I always like to win.
"Having said that we knew that Manchester United are an amazing team with quality players.
"Judging how we played in the second half I think we deserved a bit more than leaving here with nothing."
Opposing boss Alex Ferguson said the Saints were the best team to come to Old Trafford this season but Pochettino would have preferred points on the board.
"His words don't really serve as a consolation to me," added Pochettino.
"But I want to say thank you for those words and it also reinforces the w ay that we want the team to go forward."
His side travel to face current Premier League leaders Manchester United on Wednesday night, and the Argentinean is targeting all three points.
The former Espanyol coach took charge at Southampton following the sacking of his popular predecessor Nigel Adkins but won over a lot of fans with a 0-0 draw against Everton last Monday night.
Next up is a trip to Old Trafford to face United but Pochettino is not planning to take his squad there to merely make up the numbers and wants make the long trip back down to the south coast after causing a memorable upset.
"Every game is important, every single match we play is important to us," he said.
"No doubt they are going to be tough and complex games against Manchester City and Manchester United but all games are like that.
"All games are key and the glamour factor does not dictate how we play, the main thing is that we go out with a winning mentality and we want to win every single game, we always respect our rivals but our aim has to always be to get the three points."
Club captain Adam Lallana could be fit enough to make the bench for the game against United and Pochettino says he will not rest key players with one eye on a crucial relegation clash with Wigan on Saturday.
"Lallana may play, he has been training well and looking okay so he may get some minutes against Manchester United so we will see how that develops," he said.
"We're going to Manchester to win and that has to be our mentality and I'm going to chose the players that will do that for us and for the team.
"Every single game represents three points whether you are playing against Manchester United, Wigan, Man City or Newcastle."
The draw against Everton came after Pochettino had had just four training sessions with his Southampton players and they were an impressive attacking unit for most much of the game.
Since then the club have had a trip to Spain, w here they made use of Barcelona's training facilities, and Pochettino believes the Spanish trip, coupled with the gap of nine days between their fixtures, has done wonders for team bonding.
"We are definitely happy with the break (between the Everton and Man United games) and the more time that passes the more we will be able to get the team playing how we want to play," he said.
"Even against Everton with just four sessions under our belts we saw positive aspects and we just want to keeping adding more.
"We achieved everything we wanted out of the trip to Barcelona, the togetherness of the team was very important to us and the players got to know us (Pochettino and new coaching staff) and we got to know the players well.
"It is important that the players feel there is an entire solid team of people working with them at the football club and being together 24 hours of the day was important, not just three or four hours a day in a training session. "
Last week's draw at Everton leaves the Saints four points clear of the Premier League relegation zone with 15 games of the season remaining for Pochettino to secure top-flight status for at least another campaign.
Adkins was sacked by Southampton earlier this month, despite an upturn in form which saw his team come from 2-0 down to draw 2-2 at Chelsea in his final game.
Former Espanyol coach Pochettino was installed as Adkins' successor and the Argentine takes Southampton to Manchester United on Wednesday for his first away game.
"I don't know a lot about him to be honest but I will have a chat with my brother [Martin] as he's been scouting in Spain," said Ferguson.
Regarding Adkins, the United boss added: "I spoke to him the day before he was sacked. He phoned me about his situation there. I mean I can't understand these things. I think their form had picked up. Was it two defeats in 12 games? That's not bad for a team at the bottom of the league.
"Southampton's fans have got a way of demonstrating and there were a few banners up at the Everton game. It's nice in terms of recognising the contribution Adkins made. He brought them up into the division and it's not an easy job."
United won 3-2 at Southampton earlier in the season in a thrilling game and Ferguson is expecting another tough test at home on Wednesday night.
"We're not taking anything for granted," he continued. "We've got Southampton on Wednesday, then we go to Fulham and then it's Everton at home. These are big games, massive games.
"Teams will drop points [in the title race]. Not just us but Manchester City will drop points, absolutely, it's a certainty Chelsea and Arsenal will drop points.
"We'll probably drop two or three ourselves but the important thing is accepting the challenge that every game is just as difficult as the rest."
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Pochettino was part of the Argentina side beaten 1-0 by England in Japan in 2002, where his foul on Michael Owen conceded the penalty from which David Beckham secured a famous victory.
A lot of water has passed under the bridge since then, with 34-year-old Ferdinand no longer part of the international side under Roy Hodgson and Scholes, 38, already having come out of retirement once.
However, speaking ahead of tonight's Barclays Premier League clash at Old Trafford, Pochettino feels both men deserve the utmost respect for continuing to play at the highest level with United.
"I am not surprised by their professionalism and how they have maintained themselves over the years," said the 40-year-old, who will be looking to secure his first win since replacing Nigel Adkins.
"I know they are amazing athletes, they take care of their bodies and their fitness.
"As a manager, I am very happy to see them still playing, it makes me proud to see players can last that long, which is all down to their professionalism."
Former Espanyol coach Pochettino used the break in domestic action to take his players away on a short training camp in Spain, which he believes helped the players and new staff bond.
While Pochettino is determined to stamp his own mark on the team, the Argentinian remains conscious of what the squad had achieved before his arrival.
"We have our own work ethic and our own philosophy as to where we want to take the club," he said.
"We have goals and ambition, we want to work for the team to achieve those goals. However, that does not mean we want to be anything contrary to what was being done before, it is not about that.
"It is more about our philosophy, how we want to take the team forward and how we want to achieve those goals."
With defender Jose Fonte still missing through injury, new signing Vegard Forren could be set for his debut at Old Trafford after jo ining from Norwegian side Molde.
Pochettino hopes the 24-year-old, who has not played a competitive match since December, will settle quickly into the tempo of the Premier League.
"We are very pleased Forren has decided to come here. He is a player who is going to cover certain necessities of the squad," Pochettino said.
"The most important aspect now is he adapts well to the team, is up to speed and will be able to compete with us."
Southampton are currently four points clear of the relegation zone and travel to struggling Wigan on Saturday before hosting champions Manchester City on February 9.
Pochettino remains confident his current group of players have what it takes to avoid being dragged into a dogfight for survival.
"I am happy with the quality of the squad and am confident we can do well in the league," he said.
