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You need to modify the default document template to turn it off. The spell checking is a document level setting. To do this:

1) Go to OmniOutliner menu -> Preferences -> General tab
2) Click on 'Edit New Document Template'
3) With the template open, turn off the spell checking (Edit -> Spelling)
4) Save and close the file

Now any document you make from that template will have it off by default.

Let me know if you have any questions!