Celtic have signed striker Junior Adamu, winger Joel Mvuka and defender Benjamin Arthur on loan on Deadline Day.
Austrian international Adamu was the first arrival on the final day of the window, arriving from Bundesliga side SC Freiburg.
Celtic will pay a ÂŁ432,000 loan fee for Adamu, with a ÂŁ4.3m option to buy.
He started his senior career at Liefering in Austria, before moves to RB Salzburg, St. Gallen and Freiburg, where he has scored seven goals in 67 games in all competitions.
The 24-year-old, who has been capped nine times by Austria but has yet to score for his country, told Celtic TV: "I'm very excited to be here and I can't wait to get started. I want to give my best for this club and try and win the title, also the cup, and to score a lot of goals and help the team, and I also want to improve as a player.
"I've spoken with the manager and he's given me good motivation, and I trust him, he's given me that trust, he's told me how the system is and I feel the connection is really strong.
"I'm dangerous in the box and hungry for goals. That's what the manager sees in me and I want to show that on the pitch."
Celtic then confirmed the arrival of Benjamin Arthur on loan from Brentford until the end of the season.
The 20-year-old central defender made his first-team debut for the Premier League side earlier this season, playing his part in a third-round League Cup victory over Aston Villa.
Arthur, who has played for England at youth levels, added: "I am really honoured and happy to be joining Celtic. It is one of the world's biggest clubs, so it's a really exciting move for me.
"I know we have a quality squad with guys who have won so much already, and I can't wait to join them and hopefully play my part in bringing more success to the club and to our fans.
"We have a lot of challenges ahead and I am ready for them and ready to do all I can to help the club achieve all it can."
Celtic also added winger Joel Mvuka on a loan deal from Lorient.
The Norwegian, who arrives until the end of the season, became the club's fifth and final arrival during the January transfer window.
There is an option to buy the 22-year-old, who was set to join on a permanent deal before issues with his medical put the move in doubt.
Mvuka told Celtic TV: "It feels really good to finally be here. The thing that sits in my mind from playing at Celtic Park was before the game when the fans were singing You'll Never Walk Alone, and I thought 'Wow!'
"I was a team-mate of Sebastian Tounekti's at Bodø/Glimt and I know him very well so I can't wait to finally see him again.
"I'm a fast winger, I can play left or right, and I'm good with the ball, and I can't wait to get started."
Celtic boss O'Neill has held talks with free agent Alex Oxlade-Chamberlain.
The 32-year-old former Arsenal, Liverpool and Southampton player has been training with the Premier League leaders after leaving Besiktas last year.
O'Neill said: "He has trained at Arsenal for some time, since his time in Turkey.
"I would be very interested in doing it, but as I said, I am very surprised people haven't taken him up closer to the area.
"I'm sure he'll have lots of options closer to London.
"He was very excited by what we were saying and he knows about the football club.
"(Former Celtic goalkeeper) Joe Hart has spoken to him - not on our behalf - but that was a conversation he had.
"If it doesn't materialise, it doesn'tmaterialise."
Meanwhile, Celtic rejected Nottingham Forest's third bid for midfielder Arne Engels worth up to ÂŁ25m.
The Premier League club wanted to pay a guaranteed fee of ÂŁ20m for Engels with ÂŁ5m in add-ons.
Celtic mulled over the bid for their club-record signing but ultimately turned down the offer. They had previously rebuffed Forest's first two bids for the 22-year-old, with the previous offer of ÂŁ17.3m.
Celtic have had no plans to sell Engel in this window and on Sunday boss Martin O'Neill said he hoped the club would reject any offers.
Engels was signed from Augsburg for ÂŁ11m in August 2024 and has two-and-a-half years left on his contract with Celtic.
There were three outgoings from Celtic Park on Deadline Day as Stephen Welsh returned to Motherwell on loan, goalkeeper Tobi Oluwayemi moved to Leyton Orient on a permanent deal and Johnny Kenny signed for Bolton Wanderers on loan until the end of the season.
