They signed Senegal star Cisse in a £10 million move from Bundesliga side SC Freiburg in January, since when the 26-year-old has netted 13 goals in 12 appearances.
And O’Neill told the Sunday Sun: “It’s been a great season and his (Cisse’s) contribution since January has been immense.
“We were aware of him but we weren’t going to be doing anything in January.
“I needed to have a better assessment of my own team at that time but Newcastle did very well bringing him in. He’s been fantastic for them, really fantastic.”