I purchased Osciroi from the Mac App Store because the screenshots presented a lovely, weblog-like interface with the option to customize themes, and at the time I was looking for something different in the diary/journal market.
Osciroi is a reasonable diary app, but what made me stop using it is the app's editing method. Turns out that the actual writing/editing of journal entries doesn't take place within the lovely interface; when you click the edit icon the app switches to a plain-text-editor interface, where adding hyperlinks and images is unintuitive (at least, it felt unintuitive to me). I found the interface-switch to be jarring/cumbersome. When I made the purchase, the editor's interface wasn't included in the Mac App Store's screenshots, which I think is somewhat misleading.
So what does Osciroi offer? You can create different journals (they're saved as oscdiary files) and choose which to open, and also change their titles after having created them. To the left there's a small calendar, where days with entries are underlined. You can use the calendar to navigate between months/days and there's a "go to date" option if you click the month. As far as I can tell, you can create more than one entry per day. Upper left in the toolbar, just above the calendar, there are "edit" and "search" buttons. "Edit" allows you to create new entries (it's called "New Journal" and not, say, "New Entry", so it's a bit confusing) or edit recent entries. You can add images and hyperlinks to entries. There's an option to "clean" unused images. You can search entries by keyword, date or tag. Yup, Osciroi offers an option to tag entries, which is a nice feature. Above each individual entry there are an edit pencil icon and a delete eraser icon. There's an option for simple password protection. Osciroi offers several lovely themes to choose from, but the themes only effect the way you view completed entries; as I already pointed out, the writing/editing itself takes place within a plain-text editor.
I ended up uninstalling Osciroi. My current journaling app is Day One, whose minimalist, elegant interface I love. But if you're looking for a more complex app than Day One, an app that also handles images, audio etc., while still having an eye-candy interface, check out CallitADay and newcomer Per Se (from the developer of the now-open-source Journler).
All of the above reflect my personal tastes and opinions alone. Others of course might have a different experience regarding Osciroi, and I suggest that you look at other opinions/reviews before you make up your mind.