I do not think OOo is that much integrated with KDE (or with Gnome or any other of the DEs available) that it takes the prefered date/time formatting from the KDE configuration. It would be a bit tricky because it is KDE internal and KDE has the right to do it different tomorrow.
System setting imho means not KDE, but Linux. But I do not know of any mechanism there. I can also not find anything like that to be set lin the environment by /etc/profile.
When you use the OOo manual (in the Help menu) and search for Date there are several chapters about it.
For me at least that wouldn’t be enough. When my language is Netherlands, my date written in numbers will be DD-MM-YYYY or something like that. That is not acceptable for me, I use the ISO standard (which is btw also the NEN but mostly ignored) YYYY-MM-DD.
I’ve taught myself to use date fields. If I open docs in a ‘dutch’ OO.o they appear as dd-mm-yyyy, if I open them in a ‘us’ OO.o they appear as yyyy-mm-dd.
I do agree, that date should always be ISO, but I also think we should not type dates: tonight I will be seeing a play in a small theatre, the posters say friday, 18th of march !! Their flyer says the same, some other show starts at 201:15 PM.