I use OO daily in a number of ways. First, I have my To Do list in OO - I have tried OmniFocus, respect it for the elegance of its design, but the way I work requires far more flexibility (and far less adherence to GTD).
My list has items (not just actions, but also notes, reminders, odd jottings, etc.) indented under different headings (sort of like Projects), and I use color styles to flag things as overdue (on my part or others'), or things needing immediate attention.
Having them in an outline means I can easily move them around when I need to into other groupings, or nestle certain items as indents under another. I have a macro to insert a time stamp if I want to remind myself of when I called so-and-so, or ordered something, or its due date.
I also use OO as a free-form, working notepad for every kind of project management. Lists, ideas, goals, preliminary outlines for writing projects, a database to organize research (I copy and paste research into an outline so it's all in one place), etc.
Outliners are ideal for this, because you can quickly move things around, play with their grouping, promote and subordinate items easily and quickly, and collapse and expand groups to focus in on just what you want to see at one time.
Word processors like Page and Word have outlining functions, but they're very clunky compared to OO.
The great thing about a good outliner is that it's so customizable, and can be tugged and pulled to work the way you want it to. I've been a big fan of outlining since the early 90s, when a Windows program called Ecco Pro came out. It was so good it was the sole reason I stuck with Windows long after I was ready to switch to Mac. (It has since been discontinued, but there are still die-hards using it and swearing by it.)
Like OO, it was a very elegant, pleasing, reliable outliner. It did have a few features that OO doesn't have: most notably the ability to assign multiple values to an item via customizable columns) and hen the ability to filter the view based on the columnar values. This allowed you to "see" your data in different ways. For example, if it was a Project Management list, you could assign an item to several categories and several people. Then you could look at all the "Budget" Items, or just the Budget Items assigned to Fred. Etc.
I have high hopes that the next version of OmniOutliner will have comparable capabilities. If it does, there will truly be no limit to its uses.