[Brian]

Folks will want to make sure that the OmniFocus list appears in the "Mac.com" section of Reminders - a list in the "On My iPhone" section will not work for these purposes.

I'd also suggest using Settings.app on your phone to make the OmniFocus list the default reminders list. (In "Mail, Contacts, Calendars", tap on your iCloud account, then scroll down to the Reminders section at the bottom of the next screen.)

This allows you to use Siri's "Remind me to <x>" phrasing. Once you tell it what you want to be reminded about, Siri will ask you for info on when you'd like the reminder to fire.

Hope this helps!