It's probably confusing to define what the parent(s) and child(ren) are in situations like this, but here are two possible ways.
From the 1 retrieve the many:
SELECT o/id
FROM Masterdata.Organisatie o
INNER JOIN o/Masterdata.ParentOrganisatie_Organisatie/Masterdata.Organisatie p ON p/id = blah
From ony of the many, retrieve the one:
SELECT p/id
FROM Masterdata.Organisatie o
INNER JOIN o/Masterdata.ParentOrganisatie_Organisatie/Masterdata.Organisatie p
WHERE o/id = blah