The impact of the Northern Irishman’s arrival was instant, guiding the team to four wins in his first six matches, including three points over then-league leaders Manchester City.
The Belgian goalkeeper feels that the Black Cats have been a different side since O'Neill took over in December and believes that his influence is plain for anybody to see, having brought them from the bottom of the table into the top 10 since his arrival.
"There's been a big difference here since Martin O'Neill took over," he said in The Daily Mirror.
"A lot of good wins have helped the confidence too, but that can change really quickly. The main thing is to follow up because the games are coming quickly."
The 23-year-old kept the Wearside club in the FA Cup at the weekend with a sublime double save late in the game against Everton, which means that his side will get another chance to progress to the semi-final to face their opponents' Merseyside rivals, Liverpool, at Wembley.
"Heitinga's header was going just inside the post, but luckily I got a hand on it," he said.
"Unfortunately it dropped for Jelavic and all I could think of was getting up as quickly as I could and lucky for me, he hit it into the side of my body.
"I didn't know too much about it, but as a goalkeeper you try to make yourself as big as you can and luckily for me I blocked it so I made myself big enough!"How do you stay up to date with football when on the move? With http://m.goal.com – your best source for mobile coverage of the beautiful game.