WBS and OBS intersection

Engineering projects are often broken down based on the system level architecture decomposition of the thing to be produced rather than by the organization structure and skills.

I might have a hydraulic system design project that requires mechanical engineers, electrical engineers, supplier management, test and evaluation, etc. Those functions are in different parts of the organization so while the WBS of the project has a branch for "Hydraulics", the work under that branch of the WBS can involve many teams outside the OBS branch for "Hydraulic Systems" so the PM must understand where that work is actually being performed.

The reason those related tasks often don't go under the WBS for electrical, SM, etc. is that it's easier to see if you have a complete plan for the system of interest if the related tasks are together, rather than having to piece orphan work statements together and try to identify whether they form a complete plan or are there any gaps.

While the PM may be managing from a system architecture view, each of the organizations have their own people and budget, which they manage by OBS rather than WBS so different managers are looking at different views, both of which must agree.