One-To-Many Relationship Between CPTs

I am looking for a way to create a one-to-many relationship between CPTs.

Let’s say I have 4 service providers (P) in CPT-Providers.

A provider may be a member of a safety organisation (O) listed in CPT-Organisation

P1 member of O1.
P2 member of O1
P3 member of O2
P4 not member of any.

When creating a CPT-Provider, I would like to somehow create the relationship between the provider and any organisation they are a member of.

This could be a dropdown multiselect list in CPT-P, to select Os.

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Another practical example of the concept, is to show which Safety Standards (O), a restauraunt (P) complies with.

Any ideas?

Thanks

CSGL