The Omni Group

Useful Scripts is a collection of AppleScripts for OmniOutliner that will really help you perform certain actions much more quickly. They also serve as a great way to familiarize yourself with OmniOutliner's AppleScript interface in case you'd like to write a script to help with your evil plan to conquer the universe, or something.
Requirements: OmniOutliner or OmniOutliner Professional 3 or later.

This is a package of sample documents for OmniOutliner to give you an idea of what sorts of things the application is capable of. Pretty pictures, movies, and audio files make this download a bit on the large side, but we promise you'll like what you see.
Requirements: OmniOutliner or OmniOutliner Professional 3 or later.

This package adds six new export types to OmniOutliner 3 Professional: CSV, HTML (Unordered List), HTML (Headers), LaTeX, HTML (Dynamic) IE compatible, Microsoft Excel (XML) (You'll need at least version 11 of Microsoft Excel in order to open Excel XML files.)
Requirements: OmniOutliner Professional 3 or later.

Bruce D'Arcus has developed a nice plugin to export your documents to the S5 slideshow format.
Requirements: OmniOutliner Professional 3 or later.

Adam Lindsay brings us a plugin that will export your outlines to the ConTeXt text formatting language.
Requirements: OmniOutliner Professional 3 or later.

Fletcher Penney's plugin will convert to the Markdown text format.
Requirements: OmniOutliner Professional 3 or later.

This SDK (that's a TLA meaning software development kit) has guides for using AppleScript to control OmniOutliner and for building your own OmniOutliner export plug-ins. If you're the kind of person who likes to take things apart and reassemble them for new and unique uses, you might enjoy these guides.
Requirements: OmniOutliner or OmniOutliner Professional 3 or later.

Helpify is a Python script that transforms an OmniOutliner file into an Apple Help Book, complete with an index, internal links, navigation, and other niceties. The result can be dropped into your own Mac application. All current Omni Group products' help books were created with this tool. The included outline acts as documentation and as a starting template.

Rob Trew has created an app that will grab your outline from MS Word and send it to OmniOutliner or save as OPML. It requires access to MS Word.
Requirements: MS Word

Our friend rky has written a script that will take some LaTeX equation code and convert it into a PDF attachment that gets placed in your outline.
Requirements: OmniOutliner or OmniOutliner Professional 3 or later.