Last night, it was revealed the family of the 36-year-old Corkman, who enjoyed spells with Manchester United, Celtic and Sunderland, are planning to bring him home ahead of chemotherapy for pancreatic cancer.
"I gave him his debut at Celtic", O'Neill said at today's press conference.
"The news, we heard about it just recently, we didn't know how bad it was".
The Ireland midfielder David Meyler said: "Thoughts go out to him and his family, from the whole team".
"For a young man at 36 years of age, it puts things into perspective".
Miller appeared 21 times for the Republic of Ireland between 2004 and 2009 while he also played with QPR as well as Leeds United on loan.
Some incredibly sad news began to emerge last night that former Irish global Liam Miller was gravely ill, fighting cancer.
Ian Harte - who played for Ireland with Miller - said: "Very sad news about one of my old teammates Liam Miller, thinking of him and his family at this very hard time".
He tweeted: "Awful news about Liam Miller keep fingers crossed that he somehow pulls through.
Thoughts and prayers are with my old team Mate and friend Liam Miller as he and his handsome family fight through this battle!"
Jacob Burns, the former Leeds and Australia midfielder who is now head of football at Perth Glory, tweeted: "In shock!"