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If you create a blank document and open a dialog to save it as a template (File > Save As Template ...) you will be able to see where the .oo3Template files are stored.

It is likely to be :

[UserName]/Library/Application Support/OmniOutliner 3/Templates

(Rather than fully deleting a template file, it might, of course, be prudent to simply move it for safe keeping to one of your document folders).