I have been using OneNote in Windows environments for years and I always appreciate its ability to synchronize between machines.
About a month ago I had a similar need to yours because I started to work also on iMac.
There is Outline app (http://outline.ws) that not only reads OneNote notebooks but also allows editing them.
It supports native OneNote format and can easily synchronize changes between Windows and Mac OS X machines.
It also integrates with Dropbox so you can directly open notebooks from your current repository.
I am still watching carefully this synchronization abilities but it seems to work pretty well so far (I do not use Dropbox but Bittorrent Sync to move files between Windows and iMac).
Previously I had been using Outline on iOS for about a year or two in read only mode - usually I exported bunch of OneNote notebooks to my iPad just before my holidays to have them available in case I need to find some information. Outline is great in this scenario.