I find that ORS very puzzling, particularly the definition of obstacle in a):
... above the highest obstacle within a radius of 150 metres (500 feet) from the aircraft ...
If you are 450' away from any person, vessel, vehicle or structure then you violate para b) & para a) does not apply. If you are 550' away from any person, etc then para b) is OK but para a) doesn't apply anyway.
The only way that the ORS makes sense is if you interpret obstacle as something that isn't a person, vessel, vehicle or structure. Ground, water & very tall trees would seem to be the only obstacles that aren't structures.