The winger joined the Wearside club for £10million from Manchester City last August, but has taken time to rediscover the form that helped him build a strong reputation at The Etihad.
Johnson scored his third goal in eight Premier League games during his side’s 3-0 victory against West Ham on Saturday.
While many expected the England international to hit the ground running upon joining his home town club, O’Neill has revealed he was not too surprised at the winger’s initial problems.
The Irishman explained that the 25-year-old had fared well as an impact substitute at City, but was struggling to get accustomed to life as a regular starter at the Stadium of Light.
“I’ve said this before that Adam has found it difficult,” O’Neill told The Herald.
“With respect to us he was playing with some very fine players at Manchester City. He was having little cameo roles, coming on for 20 or 25 minutes and doing very well.
“Here it was totally different. He was coming in and the expectation was very high from everyone, but not really from myself. One man isn’t going to make the complete difference.”
The former Celtic and Aston Villa boss then mentioned one man, who has made a significant difference to the Black Cats, Steven Fletcher, who leads the club’s scoring charts with 8 goals this term.
The Scotland forward enjoyed a magnificent start to the campaign after signing from Wolves in the summer, in stark contrast with Johnson’s early season woes, O’Neill went on to point out:
“… in contrast to Steven Fletcher, he found it very difficult. Steven got off and running really quickly and Adam is coming to terms with all of those things,” O’Neill added.