Learn how to make and use an outline by learning to write, write and speak as one of the best things that come out of my public school education. Outlines? Seriously? Other than a digital white card, a good Mac outliner can be one of the better organizational tools you can buy.
New on stage is an outliner called NeO. It is affordable, full-featured (looks like an outliner), and can be the easiest and quickest way to create, manage, and edit an overview for organizing thoughts, setup procedures, and methods, planning larger projects, or taking meeting notes.  NeO looks like what you expect a Mac outliner to look like. Adding sections and indentations to the standard sketching method is the children's game, but the real requirement for fame here is NeO's ability to rearrange parts of an overview.
Unless you are experienced, it is best to start slowly with an overview. just use what you learned about sections, subdivisions and organize a project based on what you know about sketching.
Do you see, no?
NeO is packed with useful features that go far beyond what you learned in school. Graphics can be imported and dropped into the dashboard, as expected, but you can also customize the look with different fonts and styles.
Then it will be interesting. Neo can handle multiple columns, place inline comments, mark with colored labels, add bookmarks, insert links, change background colors for any item, even use talk about getting geeky-CSS; cascading stylesheets.
If the typical overview above seems cumbersome but usable, it stays there because NeO has geek written over it with a variety of tools that would make an Excel master spreadsheet user blush.
Outlines can be filtered, sections can be cloned, and NeO supports hoisting, of course, plus styles can be named and the app imports and exports text, RTF, Word, OMPL and can handle HTML, More and ACTA file formats.
This is not easily offset, despite the nominal price tag. It happens a lot here and it is worthy of the many five star reviews on the Mac App Store. A trial-before-you-purchase version is also available on the developer's website, but unfortunately there is no iOS version that takes sketches with you.
As for the original question, NeO may be Mac's best value in the round lines, but does not have the feature set (or price tag) of the Omni Outliner.