I use OmniOutliner 3 on iPad and tried it’s exports with DT, but the “.oo3” format will not be opened and the “.opml” format will be shown as XML file …
Please be more specific. It is not clear
- what your goal is,
- what exactly you are doing (export how, from where, to where, for what purpose?) and thus,
- what the actual problem is
- … or how it relates to this thread (which is not in the DEVONthink To Go category)
While a thumbnail is generated on iOS, it doesn’t seem like you can preview an .ooutline file like with QuickLook on the mac. So you can’t “open it” directly in DEVONthink To Go. However, you can open it from DEVONthink To Go in OmniOutliner. Tap the share button and choose OmniOutliner. Or you can access your .ooutline files stored in your database from within OmniOutliner if you add DEVONthink as a Location in the Files app.
You could always try to search for “omnioutliner” in the DEVONthink To Go Category… 
Where you will find this thread:
If you want to view your OmniOutliner files directly in DEVONthink To Go, you should probably convert/export them to a compatible format like PDF or HTML. I also remember someone somewhere writing an Omni Automation script for exporting to markdown, but I have never played around with omni scripts.
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I did not talk about DTTG.
I just create my outlines on an iPad and thought - after reading this topic - that I could probably use (see) my outlines in DT.
But DT does not understand the files from OmniOutliner, at least not those from the iPad App.
And I just posted my experience with this.
More clear now?
Yes, much clearer! 
Since you mentioned your iPad, I thought you were thinking of DTTG.
Viewing OmniOutliner files (and many other proprietary formats like scapple, mindnode or microsoft office files) in DEVONthink happens through the system-level QuickLook function, which lets you view files without opening them in an app. Your mac comes with built-in QuickLook extensions for many common file formats, but by no means all – for example not .oo3 or .ooutline files, as that is a niche proprietary file format.
The QuickLook extension for OmniOutliner files is bundled with the app. So you need to have OmniOutliner installed on you mac to be able to preview them in DEVONthink. You don’t even need to export/convert them, just import (or index) the file you created on your iPad.
(You don’t need to buy it if you only use OmniOutliner on your iPad. You can just install the app and let the trial expire, it will still let you view files.)
… Unless you updated to macOS Sequoia. As covered upthread, Apple changed how QuickLook extensions work, and there is a problem with OmniOutliner files for now. I imagine Omni will be quick to fix it though.
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Oh, that’s fantastic!
Great many thanks for the explanation
Regarding the TOC inspector and Markdown, it’s handy for more than just a TOC of the document.
If you want a “map of contents” sort of document in DT, the TOC feature can give you some structure.
That can also fill in a gap. Devonthink documents can’t have subdocuments, but you can link to other documents. Write your notes, then maybe put three dashes to draw a horizontal separator, and add a list of links. Categorize them with “#” headers and navigate with the table of contents if it gets long.
Or, put all that in a Markdown annotation. Which, of course, has its own TOC inspector, and on, and deeper, and through as many twists of the rabbit hole as you’d like.
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