Would or will. A question is about a certain future event
Solution 1:
"Ohad knew he had a few hours before they would open a wormhole to Mumbo Booboo."
The sentence describes events taking place in the past. The "would" acts as the simple past of will here.
https://www.dictionary.com/browse/would?s=t
As @Phil Sweet commented, "Ohad knew he had a few hours before they opened a wormhole to Mumbo Booboo" would be more common.