Vertical alignment is not an option offered by OmniOutliner but I’ve made note of your request. The only options I can suggest involve manual fiddling with either line breaks or line spacing.
Adjusting every cell manually is a lose. Seems to me that vertical orientation should be as simple to implement as horizontal. If user confusion is an issue, make it only editable using the inspector. But it definitely needs to be supported and saved in styles/templates.