I can’t tell you much about Omni. I had the version that came with my Mac but didn’t find any use for it (which does not say it is useless, it just didn’t fit with my workflow).
But you mentioned using Scrivener for journaling, as a notebook etc.: You can do that, of course, but keep in mind that the idea behind Scrivener is to produce THE TEXT:
You collect all the material you have in a project, your own notes, reference texts from other writers, pictures, videos, whatever. And the text itself of course, parts of it maybe re-written over and over again.
You work the way that comforts you most. You could outline first, say, with the index cards and then fill every card with content. Or you just write and write and then start to chop your text and rearange the order of the text bits. Maybe you mix both methods.
Whatever you use and however you work, the idea is that one beautiful day you hit the compile icon (keyboard shortcut also works, menu too) to finish your work in Scrivener and to give the text a finishing in a word processor or let an editor do his job …
But when you write a notebook in Scrivener you just mimic a paper notebook as Scrivener aims at a linear text. And in my opinion by doing so you would sacrifice a lot of the benefits a computer could give you.
This is not at all an anti-Scrivener statement, don’t get me wrong. Like you said—and to return to the subject of your thread—Scrivener is a great outlining tool, even to a point where it might make a dedicated outliner like OmniOutliner superfluous.
But a notebook or a journal does not need no outlining. There is only one real order of entries: by date. The interesting part begins where you leave that order, when you collect items to groups, tag them etc. When you sort them in non-linear ways, when items are in more than one place.
True, Scrivener has labels and stamps and the awesome new collections are actually groups that allow a look at certain elements in a non-linear way. But these features still are made to reach the one goal. Dedicated information managers/notebook apps are not made for this purpose and because of that they give you more freedom of sorting your notes in numerous ways.
These are two very different usages and so I’d recommend you to give some of these programs—as companions to Scrivener, of course—a look. Like DevonThink, EagleFiler, Together or if you like Wikis, VoodooPad. And lots more. You will find many threads about this kind of applications in the Software by Other Folk sub-forum here.