Wales triumphed 1-0 in Georgia despite being without Bale due to a calf injury that will also rule the Real Madrid forward out of a Group D showdown with Ireland, who beat Moldova 2-0, in Cardiff on Monday.
O'Neill feels the absence of right-back Coleman, who broke his leg when the nations last met in March, is just as big for his team.
"I think that Wales would naturally miss a player of that calibre. We've got a world-class player missing ourselves in Seamus Coleman," he said.
"In my mind, he's as big a loss to us as he is to Wales. Gareth is a world-class player, and he's a major player.
"Him not playing helps, but Wales have been able to win matches without him. I'm sure their mindset was pretty strong."
A first-half double from Daryl Murphy secured Ireland's victory over Moldova in Dublin, with the result setting up a crucial trip to Cardiff.
While Wales need just a point at home to secure second place, the visitors must win to keep alive their dream of reaching next year's World Cup.
O'Neill always expected a close group to go down to the wire and he's more than happy to have their hopes resting on the outcome of one game.
"Regardless of their result in Georgia, we had to win in Cardiff," he said.
"A couple of early goals settled our nerves. We had to get a result here.
"Now we have to go and apply ourselves [in Cardiff], play strongly and resolutely and I think we can win the match.
"It was always going to be like this. If we said to me that we'd have to go to Wales and win at the start, I'd have taken it."