The 25-year-old penned a four-year contract with the Black Cats after moving from Manchester City for an undisclosed fee on Friday.
“Adding quality players to the squad has been our main aim this summer and Adam certainly fits that bill,” said O’Neill.
“He has terrific ability, great delivery and I’m sure he is a player who will excite our fans. I couldn’t be more delighted to have him at the football club.”
The England international started his career at Middlesbrough before moving to the Eastlands outfit in 2010.
Johnson, who scored 11 goals in 73 appearances for City, said O’Neill’s influence was a major factor in his decision to spurn the advances of others clubs and join Sunderland.
“It’s great to have signed and it’s a fresh start,” Johnson said. “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.”
“I want to perform on the pitch for the fans and I’m just looking forward to getting out there now.”
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