Houllier revealed he rejected other offers before taking charge at Villa and hopes to build on the success achieved under O'Neill.
He said: "It was really starting to tickle me. I wanted to go back pitchside.
"I had, of course, other proposals but this one seemed to be the most interesting in terms of working conditions and environment.
"It's a club that according to me has belonged between seventh and 12th place in the Premier League.
"But, above all, it's a historic club which has won the European Cup.
"It's a club which has got good infrastructure and a fine team which is after one thing - to improve year on year."