Well, it is supposed to work this way.
You select a file (either by tap holding it or using the (...) menu at the top right and entering select mode), then tap the parachute icon at the bottom. A small gray parachute icon will appear next to the file name.
Or you can choose the parachute icon when viewing files to filter for just the offline files.
However, every time I do this, and then go into Airplain mode on my iPad Pro, all the files disappear anyway. This may just be a problem with the Pro, so I suggest you try it for yourself.
The Office apps themselves tend to retain the most recently edited files, and I could work on several in Airplane mode (when I accessed them through the "Recent" icon/view). However there does not seem to be a way to force a local copy, and the only way I could tell if a copy was local was to try and open it.
You can, of course, create and save documents locally by choosing the iPad instead of One Drive when working in the Office apps, but then you can't edit it anywhere but the iPad.
Office gives you plenty of oportunites to move a file to the Cloud (One Drive), but there is no method to move them back to the iPad, nor can you move them to another cloud service.
All in all, Office 365's cloud services on iOS are half baked; only good if you work almost entirely through One Drive, and mostly online. They are also inconsitent with the One Drive app. I could often access offline files in Word, but not see them in One Drive.
Note: I did almost all my testing in Word. I'm just assuming the other office apps work the same.