Bale was left out of the Wales squad to face Northern Ireland and Ukraine as he recovers from a calf injury, while they will also be without Arsenal midfielder Aaron Ramsey due to a thigh problem.
O'Neill was hoping the match against a Bale-lead Wales would provide a perfect test for their daunting Euro 2016 group, where they were drawn world champions Germany, Poland and Ukraine.
"We know that when we go to France we're going to play against some very good teams and teams with some exceptional individual players which Gareth Bale is - Aaron Ramsey comes into that category too," the 46-year-old said.
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"It would have been good for our preparations to have to deal with that but equally the fact they are not playing sets us a different challenge. It's been difficult to give the players as much meaningful information as possible because of their absence.
"When you look at Wales over the qualifying campaign, you see how influential Gareth Bale was in terms of his number of goals and assists. Finding footage of Wales without Gareth Bale is quite difficult. We think we've given the best information we can.
"It would be great if he had have been able to play. Chris [Coleman] is like me, he probably knows the vast majority of his squad that are going to go to the tournament, I'm sure the players that come in will want to stake their claim for an opportunity in the summer as well."