However, how to change the display order of goals? As dragging _Start further down will just put it as a sub item to another goal?
That is not possible, they can only be displayed in alphabetical order.
Thanks Tinkerer

. I've been reading up further on Osiris earlier today and think I'm very close to grasping the full structure now

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Event driven, order doesn't matter in Story Editor as it's all alphabetical. Just with the caveat that sub items of goals won't be active until the parent goal completes.
What's been throwing me is how the default order of goals in the Story Editor (basic level) isn't completely alphabetical. (e.g. __Start being before __GMStart). Would be nicer if the Story Editor had an improved UI / UX :), especially as execution order is a common issue for new modders =/.
Casmus88 had a wonderful post here that I found which goes into more detail about execution order and info on how to resolve, which I'll be able to use for future occurrences of issues like this

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http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=627657