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You can use OmniFocus on your iPhone without owning a Mac. You will only be able to view your data on your iPhone. There is no way to sync with any application on Windows.

There are also a few features that are not available (yet?) in OmniFocus on the iPhone, including:
1) configuring repeating actions and projects
2) reorganizing existing projects into folders. (You would have to delete the project and create a new one inside the folder.)