Cox reached Judges' Houses in the 2011 series, but was snubbed for the live shows by Gary Barlow.
Frankie Cocozza, Marcus Collins, Craig Colton and James Michael ended up making the final cut, ahead of Cox.
Cox claimed that the rejection was the "worst feeling", but said that over the year he had come to appreciate Barlow's decision, arguing that the Take That star had done him the "biggest favour".
"I felt like a nervous little boy last year," he said. "Now, I've hopefully come back as a man."
Following his rendition of Katy Perry's 'The One That Got Away', Nicole Scherzinger declared that Cox had an "excellent recording voice".
Louis Walsh commented: "Gary did the right thing, you're stronger and you're better. You've absolutely convinced me there that we've found somebody brilliant."
Meanwhile, Tulisa Contostavlos claimed that if she'd been mentoring the Boys last year, she'd have taken Cox into the Live Shows.
Gary Barlow concluded: "Of the whole series, yours was the hardest no by far. I really wanted to see you develop. But you've come back as you." Cox got "four big yeses" from the judges.
The X Factor continues next Saturday at 8pm on ITV1. The Xtra Factor airs afterwards on ITV2.
Watch Joe Cox's Xtra Factor audition: