The 32-year-old former Liverpool man, who is currently with German side Wolfsburg, followed Manchester City full-back Wayne Bridge into the Stadium of Light.
O'Neill said: "Sotirios offers a real physical presence, coupled with a wealth of experience, which will be an undoubted benefit to us in the next few months.
"I'm delighted to welcome him to the club and I'm sure he is looking forward to the challenge of returning to the Premier League."
However, unlike Bridge, whose deal was completed before the noon cut-off point, the 6ft 4in Greece international will not be available for tomorrow night's Barclays Premier League clash with Norwich.
Kyrgiakos began his career in his home country with Panathinaikos and spent two of his initial seven years in Greek football on loan at Agios Nikolaos.
After winning his domestic double in 2004, he joined Scottish side Rangers and collected league and Scottish League Cup winners' medals in his first season.
Kyrgiakos joined German outfit Eintracht Frankfurt in 2006 and had a season back at home with AEK Athens before heading for Liverpool, where he made 30 league appearances, during the summer of 2009.
Bridge, meanwhile, is relishing the chance to play football once again after completing his move.
The 31-year-old former England international has only one appearance and 78 minutes of football to his name this season to date and told SAFC TV: "I'm delighted to make the move. I have met the manager and the lads and everyone seems great.
"I'm just excited to start playing football and playing a few games. The club is on the up - the manager has come in and done very well.
"Just coming in and speaking to him, everything has been positive. I have trained with the lads and they are a great bunch who are feeling really positive.
"Hopefully I can help the club progress even more."
Bridge's only game this season came in a Carling Cup third round tie against Birmingham on September 21, and he admitted he cannot wait to get started on Wearside.
He said: "I'm grateful for the chance and want to go out and work hard.
"I have looked at the fixtures and this month's quite busy with a game every few days. That's going to be good for me."
Bridge and his new team-mates will attempt to gain revenge over the Canaries tomorrow night after slipping to a 2-1 defeat at Carrow Road in September.
For O'Neill, it will provide a chance to go head-to-head with one of his proteges.
Norwich boss Paul Lambert played under O'Neill during their time together at Celtic, and indeed, was given time to study for his coaching badges in Germany by him as he started to consider where his future career would take him.
Lambert is one of a series of men to have worked with O'Neill who have since joined him among the managerial ranks, and the Black Cats boss is delighted to see them thriving.
He said: "It's particularly nice to see some people like Paul, Neil Lennon, Simon Grayson, who I have had as players, it's pleasing to see them doing so well.
"I had Paul in the latter stages of his career, but he was far from finished. He went on to win some big matches for us and some big games with us.
"But it was just in his final year there when it looked as if he was going to be looking at either coaching or managing that he asked me if he could take his coaching badges in Germany, where he had a great affinity with them in his time with Dortmund. So I said 'yes, absolutely, go and do it'.
"He went over there, did the badges and then I realised this was what he wanted to do, coaching or managing, so he stepped into it and I think he has taken to it very, very well indeed, as results would suggest, and well done him."
However, for all that O'Neill and Lambert have a history, the older man will be expecting no favours at the Stadium of Light tomorrow night.
He said with a smile: "Absolutely not, not at all."
PA
Comments