I have seen various OOMs caused by the Backup service.
Or rather, a very large calendar database causing the calendar export that happens during backup to eat up lots of RAM causing OOMs.
Fix here was to clean out the calendar by removing the corresponding account (Google) and adding it again.
So such OOM phenomena can appear with “no apps open”.
True though that OOMs appear in journal (and dmesg), so this issue appears not to be an OOM one.