It is pronounced "Wah-bash," just as you would say if it were written "Wabash." Guess my friends are Francophobes, or something.
You do know that Ouagadougou is the capital of Berkina Faso.
What? You didn’t know there was a country called Berkina Faso?
(Why did I insert that in this post? 1) Because Ouagadougou is also of French derivation and the "Oua" is pronounced as it is in Ouabache; and 2) For your edification. You have learned a geography fact of which you were unaware, haven't you?)
Ouagadougou is pronounced, roughly, "Wah ga da ga."