I have no fresh onions on hand, should I use green onions or organic onion powder?

Agree with HalfPint.
If the green onions are fresh, use the same volume as the other onions in original recipe.
Is you go for onion powder, use much less...only about 1/4 the volume if onions in original.
If you want to replace the lost vegetable volume, chop something like celery or fennel and add it.

Either will work. I like green onions because of it's mild flavor.