Siri puts things into Reminders; you'll have to sync your Reminders with iCloud and launch OmniFocus so it can capture them from iCloud. (We don't have access to the local Reminders database on the device.)
via www.omnigroup.com/blog
'OmniFocus for iPhone' (15.99 €; App Store-Link) schnappt sich über Siri eingesprochene ToDos aus der iCloud, die Apples 'Reminders'-App