The boyhood Newcastle supporter was booed by a section of the home crowd when he came on as a second-half substitute during former club Swansea's 0-0 draw with the Black Cats at the Stadium of Light on Tuesday night.
However O'Neill believes the 27-year-old will swiftly win over his critics.
He said: "In the scheme of things, it's how Danny does for the football club that will determine everything else.
"I have got really no doubts that he will score goals and he will prove a great buy."
Graham, who signed a three-and-a-half-year deal on Thursday afternoon after Swansea finally accepted Sunderland's £5million bid, was the last of three men to join the club in January.
He follows in the footsteps of midfielder Alfred N'Diaye and defender Kader Mangane, who will spend the rest of the season on loan from Saudi Arabian side Al Hilal.
O'Neill was happy with the business he did and is expecting to see a return on his investment.
He said: "I am pleased. I think Alfred N'Diaye will eventually - when he learns the game in the sense of the things we expect of him to do that might be a little bit different to his previous club - will be excellent for us.
"I think Danny will do well and score a few goals as well, which will help and big Mangane, if he gets an opportunity, I am sure between now and the end of the season will prove some value as well."
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