The 25-year-old, a £12million acquisition from Wolves, has announced his arrival on Wearside in style with four goals in his three Barclays Premier League games to date, and he will hope to add to that tally when Wigan head for the Stadium of Light on Saturday.
However, Fletcher is the only member of the Black Cats squad to have scored a league goal so far this season and while O'Neill is delighted with his return so far, he does not want his team to develop the same kind of over-reliance on the Scot which predecessor Steve Bruce's men had with Darren Bent.
The manager said: "I wouldn't want to wish that on anyone for the whole of the season, but he's an excellent finisher.
"He has had a great start to his career here, which I am delighted about. It makes things a wee bit easier to settle in, and he has done so very, very well.
"But eventually, we will need other players to score goals and I am sure in the course of time, they will do.
"But making judgments after just four games might be a wee bit rash."
The need for Fletcher's recruitment is illustrated by last season's goalscoring figures: loan signing Nicklas Bendtner, midfielder Sebastian Larsson and Benin international Stephane Sessegnon finished the campaign tied together at the top of the stakes, but with only eight each.
Bendtner has since left the club, while neither Larsson nor Sessegnon has registered so far this season with James McClean with three and Craig Gardner having claimed the four goals their side has scored in the Capital One Cup.
Sessegnon in particular is a man Sunderland believe should find the back of the net on a more regular basis, and his developing partnership with Fletcher is likely to be key to their hopes of progress this season.
Asked how long it might take for the relationship between the two men to blossom, O'Neill said: "It varies.
"You want players to be able to settle as quickly as possible, and that he [Fletcher] has certainly done.
"He has been aided greatly by scoring goals, so he feels confident now and that's very, very important.
"Over the course of time, there are going to be a number of partnerships that we are going to have to rely on.
"Sessegnon's fitness is improving. He worked exceptionally hard for us the other evening there when Lee [Cattermole] was sent off, going back and playing in really an unaccustomed position.
"Certainly it's not something he wants to do, defending right wing, but he did that very, very well for us and helped us get a result."
The fifth red card of Cattermole's career on Wearside, which came as the result of a poor challenge on MK Dons defender Adam Chicksen during a 2-0 cup win on Tuesday evening, means he will start a three-game ban on Saturday, while England winger Adam Johnson, who was used as a substitute, is struggling to shake off a thigh problem.
However, whichever team O'Neill sends out on Saturday, they will kick-off determined to end their wait for a first league win of the campaign.
They have drawn all four games to date, and while three of them - against Arsenal, Swansea and West Ham - have been played away from home, the clamour for a victory is growing by the week.
O'Neill said: "We have to work on a lot of things. I think we can improve - we would need to improve, obviously, throughout the whole season.
"I think we are capable of doing that and hopefully with that improvement, you would see us trying to win a couple of games."