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Option + <other key> in OS X is used by the text input system. It means "type an alternate character". For example, option-8 gets you a bullet point like this:

and here's "option-q,w,e,r,t,y": , ∑, , , ,

Since those combinations are used as part of the text system, OmniFocus can't use them for the quick entry shortcut. It only gets a chance to handle the keyboard event if the app you're running declines to do so.