The England winger signed a four-year contract with the Black Cats on Friday after struggling to secure a regular starting spot during his time at Manchester City.
The former Middlesbrough youngster was also linked with the likes of Liverpool and Tottenham Hotspur, but explained why he plumped for the Stadium of Light.
"It feels fantastic [to be here]," he told the club's official website. "I was born here and there have been rumours in the past about me coming here.
"It's great to have signed and it's a fresh start.
"Coming back to the North East and Martin O'Neill were the two main factors, especially the gaffer and listening to what he had to say to me.
"He made it clear just how much he wanted me to come here. To know that and feel wanted and appreciated made me feel that it was the right choice to sign.
"He knows exactly what he wants from his players and from me personally. Hopefully he'll give me plenty of confidence as well."
O'Neill, who has also added Steven Fletcher to his squad, labelled the capture of Johnson as a 'real coup' for Sunderland due to the competition for his signature.
The Black Cats boss said: "To get Adam, it's really, really big news for us, big news. It's a real coup.
"He's a fantastic footballer and he's really exciting. I just think if he stays clear of serious injury, he will be great for us, really great.
"I am pleased he has chosen to come here because at Manchester City, he might not have been playing all the time there and maybe it seemed as if he might be leaving.
"But to come to us instead of a number of other teams who were definitely in for him, I was just very, very pleased, pleased for everyone."