Before the auditions began at Wembley Arena it was revealed that Nicole Scherzinger will look after the Girls, while Sharon Osbourne is mentoring the Overs.
For his last ever series as an X Factor judge, Louis Walsh is looking after the Boys.
Gary Barlow has the Groups, a category which he admitted he "didn't expect" to mentor.
Another change to the Bootcamp format saw the introduction of the seat challenge forcing the judges to shortlist six contestants from their own category as the hopefuls stand on the stage.
Melanie, 19, is again competing in the Girls category in what is her fourth appearance on the show.
She first auditioned aged 14 in 2008, making it to Bootcamp.
In 2011 she made it through to Kelly Rowland's Judges' House but was unsuccessful. Last year she got through to the Bootcamp stage of the competition.
Before performing her rendition of Katy Perry's 'The One that Got Away', Melanie was brought to tears as the pressure of singing in front of the judges plus the 4,000 strong audience became too much.
But after some encouraging words from Nicole, Melanie got through her audition and once again brought Louis to tears.
He told the Dubliner: "You had me from the very first lyric and I think you are the one who got away."
All six seats were full and Nicole had to decide whether or not to take her at the expense of someone else.
Wow - what pressure you have got, you have seen these chairs fill up today, you have got 4,000 people watching every note you sing. Well done for doing what you have done.
Gary Barlow to Melanie McCabe
Before giving Melanie a place in her final six, mentor Nicole told her: "In your past performances, I just didn't know if I was with you, or if I felt it, or if I believed it - but you had me on this one."
Also winning a place at Nicole's Judge's House was Hannah Barrett, 17, who earned a standing ovation from the judges following her rendition of Etta James' 'I'd Rather Go Blind'.
"Hannah, you are so blessed," Sharon told her. "That was stunning."
"That's how to fight for your place in this competition - that was amazing," said Gary.
Joining Melanie and Hannah at Nicole's House are supermarket checkout girl Abi Alton, 16-year-old student Tamera Foster, returning hopeful Jade Richards and hotel housekeeper Relley Clarke.
Meanwhile Sharon began to fill the spaces in her Overs category and the first hopeful to take a seat was 35-year-old prison guard Sam Bailey.
Sam sang Emeli Sande's Clown for the Judges, with Gary commenting: "That was absolutely note perfect, absolutely incredible."
Louis added: "To think that you have been working in a prison all these years, you should have been in a recording studio.
Sam's category mentor Sharon said: "I just want you to know that all the years that I have been involved in, and not involved in, this show I have dreamed of having a contestant like you."
Fan favourite Joseph Whelan was also given a spot on team Sharon, but until the last performer takes to the stage, no-one is guaranteed a place at Judges' Houses.
The fate of the remaining contestants will be revealed during the X Factor on Sunday from 7.30pm on UTV.