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OmniFocus syncs every hour whether it has changes to sync or not. OmniFocus will also sync within about 2 minutes after you make changes. And as CatOne mentioned, it will also sync on quit if it has any unsynced changes.

So to have changes not get synced, you'd have to make the changes and then quickly force-quit OmniFocus, or quit and then choose to cancel its sync-on-quit. In other words, you'd have to be trying to block the changes getting to your iPhone.

(All of this assumes your Mac has internet access of course!)