The Scot has grabbed four goals in three games since his summer move from Wolves, helping the Wearside club to remain unbeaten in the league.
And the former Celtic manager believes the 25-year-old will add an extra dimension to the Mackems' team beyond just his goalscoring ability.
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"He is playing brilliantly for us. He has helped us immensely in terms of the goals, which is great because he is the only one to score," O'Neill told reporters.
"But the extra part of his game, which we didn't really have last season, he is bringing players into play, he is giving us that little bit of respite at times and he is proving himself an all-round top-quality centre-forward.
"He will prove real value for money."
Midfielder Jack Colback also hailed his new team-mate, underlining the former Scotland international's deadly eye for a goal.
Colback said: "In the Barclays Premier League, it's hard to score, so it's great to know we have a player of his calibre.
"If you give Fletch [Fletcher] the right service, you know he's going to finish the chances off."
Sunderland have not recorded a league win since the disappointment of an FA Cup quarter-final defeat to Everton last season, a run which spans six months.
And O'Neill is keen to end that hoodoo and believes the confidence which can be gained from meeting MK Dons in the League Cup could help.
"I know this is separated by two seasons, but we just need to win," he added.
"It will be a difficult game for us [against MK Dons]. We would like to try to stay in the competition if we can."