Apparently, for Koha's recorded history, MARCOrgCode has always defaulted to OSt. (See commit f8e9fb6445dadbdef91e13a253c9c6bcb75e0eec) I never really thought about it too much but I just took a look at https://www.loc.gov/marc/organizations/org-search.php and OSt is "Stow Public Library" in Ohio in the USA. Really we should default to an empty/blank value rather than a wrong value.
It's kind of interesting seeing how many Koha libraries around the world have a 003 of OSt...
Likewise the DEFAULT framework should not have 003 as mandatory...