Speaking at his press conference this morning ahead of the second leg of Ireland's World Cup play-off against Denmark, O'Neill offered his support to the former midfielder.
"The news - we heard about it just recently, we didn't know how bad it was - and obviously all thoughts go out to him and his family".
The Ireland midfielder David Meyler said: "Thoughts go out to him and his family, from the whole team. A very young man, 36 years of age, it puts things in perspective anyway".
Miller then joined League of Ireland outfit Cork City in 2015 before making the move to America to play for third-tier North Carolina-based side Wilmington Hammerheads in February 2016.
Fellow Irish global Noel Hunt added: "Such sad news about my old Room mate Liam Miller, Pray to God he pulls through this battle with cancer, my thoughts and prayers are with him and his family".
"Thoughts and prayers are with my old team Mate and friend Liam Miller as he and his lovely family fight through this battle!"
"The thoughts of everyone at Manchester United go out to Liam Miller and is loved ones at this hard time" a tweet from United read.
"Awful news about Liam Miller keep the fingers crossed that he somehow pulls through", McManus tweeted.