The two sides go head to head on Monday at the Cardiff City Stadium in a bid to seal one of the top two spots in their World Cup qualifying group.
Ireland captain Coleman has been out of contention since suffering a double leg fracture in a challenge by Dragons defender Neil Taylor when the sides met in a goalless draw at the Aviva Stadium in March.
Wales talisman Bale, meanwhile, is out with a calf strain, and when asked how much of an impact being without Bale through injury could have, O'Neill told Sky Sports News: "Wales would naturally miss a player of that calibre, but we've got a world-class player missing ourselves in Seamus Coleman.
"He's been missing and in my mind, he's as big a loss to us as Gareth Bale is to Wales. But Gareth Bale is a world-class player and of course, he's a major threat.
"Him not playing I think helps, but Wales have been able to win matches without him, so I think their mindset is still pretty strong, but so is ours."
Both nations can still qualify automatically for next year's finals in Russia, although they are relying on Group D leaders Serbia slipping up at home to second-bottom Georgia.