How can you trasnlate OR associations in ER diagrams? For instance, you've 2 organizations which have some reference to the 3rd, only one of these might have that relation?(ex. BussinessOwner or PrivateOwner holds only one Property) ?

Option 1 (simpler): Two nullable foreign secrets, BusinessOwnerID and PrivateOwnerID.

Option 2 (more stabilized): Both BusinessOwner and PrivateOwner are Proprietors, which means you might make the owner entity with whatever details are common between BusinessOwner and PrivateOwner. Adding an OwnerID to both BusinessOwner and PrivateOwner tables.

User an owner and Type Area. Not 2 owner Fields.

Proprietors are People entity. Types are Type entity or strings ("Business" Owner or "Private Owner")

Let's say you give a new type... "Sr. Partner" Together with your model you would need to add new filed and recode your company rules.