More than 40,000 fans are expected to attend the event, which will also see performances from acts including Metallica, Slayer, Pantera, Halestorm and Anthrax.
Osbourne is only expected to sing four songs because of health issues and Iommi explained that rehearsals had been hard on all of Sabbath's classic lineup.
"I wouldn't say it's been easy, it's been tough, because none of us are getting younger and to stand there for a couple of hours is tiring," he said.
The supporting bands had been good to work with, though, and there were "no egos", Iommi stated.
Although they are a long way from their early days in Birmingham, Sabbath - formed in 1968 - still remember the city fondly and talk about their memories.
Iommi said: "I can't remember what happened yesterday, but we can remember what happened in those days, where we used to go and the gigs we did."
That made the farewell in Birmingham extra poignant, he said, adding: "This is totally different from anything else we've done, you know we've played for 300,000 people but this is nerve-wracking."
There was also the thought that there would be no more comebacks after this.
"We're never going to do this again, this will be it," he confirmed.
He said he hoped the gig would finish things on a good note and, when asked what he thought the legacy of Black Sabbath would be, said it would be the bands that followed in their footsteps.
Their legacy would continue through them, he said.