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Besides the obvious (buy OF for iPhone), for text items you can use the email service. Send an email to that address, in plain text, with the subject line as the action name and the body of the message as the note. In a few seconds, you'll get an email back with a link that, when tapped on a device which has OmniFocus installed, will open OmniFocus and add that action via the Quick Entry form.

You can see some screenshots in this thread: