I discovered two outliners in the Mac Apps Store. One is called "The Outliner." It's a very rudimentary two-pane outliner. I cannot recommend it, especially also because the support site does not seem to be operational. It costs $1.99. I paid it, but it does not seem to me to be worth the price.
The other is called OutlineEdit. It is a slick one-pane outline with a number of extras, the most important of which is the so-called category filter. It allows you to assign different categories to outline items by giving them the same color. Look at the Website for an introduction. It costs $16.99, but it may well be worth the money. In any case, I am tempted. And not only because it looks good. I am not sure, however, that it is better than Outlinely which now also can do tags that can be filtered.
1. A tag starts with a "@" character (which I prefer over assigning a color because it is easier to type.