The Scot plundered his fourth in three games in a red and white shirt at West Ham on Saturday and for all but the dying seconds of the game, looked to have fired them to a first Barclays Premier League victory of the campaign until Kevin Nolan's last-gasp equaliser.
But while his lethal touch in front of goal - he scored both in a 2-2 draw at Swansea and put Sunderland ahead in their 1-1 draw with Liverpool - has shown exactly why O'Neill was happy to pay so much to secure his services, the former Wolves frontman has also given his side a new dimension.
His manager said: "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.
"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 and his team-mates have been hugely impressed by Fletcher's contribution to date, and are keeping their fingers crossed that he continues in the same vein.
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 the right service, you know he's going to finish the chances off."
The striker's four goals are the only ones the Black Cats have scored in the league so far this season and that is a concern for O'Neill, who is hoping the likes of fellow summer arrival Adam Johnson, Stephane Sessegnon, Sebastian Larsson and James McClean can help out on that front.
Indeed, the only game in which the in-form marksman has not registered was his debut appearance, a 2-0 Capital One Cup second round victory over League Two Morecambe on August 28, when he was still searching for match-fitness after missing much of Wolves' pre-season campaign.
Sunderland return to action in the same competition on Tuesday night when they make the trip to League One MK Dons hoping to prolong their stay in the competition after passing up the chance of cup glory last season.
It was the FA Cup which provided the excitement last season and in March a side rejuvenated by O'Neill's December arrival ran out at the Stadium of Light for a quarter-final replay against Everton firmly believing their time had come, only to lose 2-0 in tame fashion.
The Ulsterman admits the disappointment of that night may still be having an effect, with the club having won none of the 12 league games they have played since.
O'Neill said: "I know this is separated by two seasons, but we just need to win.
"It will be a difficult game for us. We would like to try to stay in the competition if we can."
O'Neill fielded a strong team against Morecambe - Fletcher and Johnson both started - but faces a series of selection decisions this time around.
Johnson is back in training after the thigh strain which kept him out of the game at Upton Park but is unlikely to be risked, while Fraizer Campbell was sent for X-rays on a foot injury afterwards and Larsson limped off with a hamstring problem.
In addition, O'Neill is unsure if central defender Titus Bramble will be able to manage a second game in four days after his lengthy spell on the sidelines.