Morello said he was approached by Ozzy and wife Sharon about putting the event together "a couple of years ago".
"I basically sat down with those guys, and made a list of sort. My first call was to Lars Ulrich of Metallica," Morello said.
"I thought, if I can lock in Metallica, and then I'm going to be able to say it's Ozzy's last show - it's Black Sabbath's last show and Metallica are playing.
"From that point forward people are going to pick up the phone."
During the event, Osbourne, 76, who has Parkinson's disease, sang while seated on a black throne - a piece of stage furniture that has since become coveted.
He said before the event: "All of us took pictures sitting in it - I've got a great picture of me sitting in that throne.
"I'm not sure what happened to it, but hopefully there'll be some Birmingham museum of Sabbath and Ozzy Osbourne, where it should be in the front and the first thing you see when you walk in."
The show's bill also included fellow rock legends Guns N' Roses, the Rolling Stones' Ronnie Wood and Aerosmith's Steven Tyler.
Morello said after the bill was announced, he was called by Guns N' Roses vocalist Axl Rose.
He said: "The phone rang because a couple of the guys from Guns N' Roses were playing. But then Axl Rose was like: 'Well, I want to play - why don't we just be Guns N' Roses?'
"So I got a number of calls afterwards from a huge bands, saying 'can we please be on?'"
Speaking about the positive reception the event had, Morello said: "I think that they were very, very aware that the world was paying attention to this moment.
"Not just the heavy metal world, but like the world was paying attention."
He added: "Heavy metal is the music that made me love music. Black Sabbath is the band that invented heavy metal music, and it is a band that is so meaningful to so many of us, and the DNA of Black Sabbath is everywhere.
"Those are things that originated in your city. Those are things that originated with Black Sabbath and Ozzy Osbourne."
"That band, and that music has meant so much to me, and it was a real honour to be any part of it."