A button to toggle between year view and month view would offer a quick way to plan for future months and consider several months at a time when looking at days of the week, etc.
major task
I would propose a different approach to this. Instead of a toggle, you could provide an "extension", à la Globaltime. One of the two features of Osmo that I found very useful (the other being the date calculator), is the Full-year calendar (see attached). This is what I usually fire up whenever I need to do planning that involves more than two-three months. Orage could do just about the same: Have a menu item that opens a new dialogue containing all 12 months of the year and some controls (current year, prev, next). This would be very useful and, I imagine, simpler to implement than a toggle in the existing UI of Orage.
Created attachment 7036 Osmo Full-year calendar
The orage project is not maintained anymore and has been archived. Closing bugs.