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Open a project, choose Export, format "Web Page", choose a new directory somewhere (say on your desktop). After it completes, drag the new folder into the trash, and empty trash. You'll see a bunch of errors that files could not be deleted because they're still in use.

The only way I can delete those files is to use Finder > Secure Empty Trash, or quit OmniPlan and do regular Empty Trash (or use terminal to rm -rf the entire directory).