You can map each unidirectional one-to-one association to a reference key. The foreign key should have the same direction as the association.
In the following example, there are two entity classes, Person and Account, and a one-to-one association between them. The association is unidirectional and navigates from the entity class Person to the entity class Account.
The association and the reference key are linked in the Mapping Editor.
For a bi-directional one-to-one association, you also just can map it to one reference key. But the reference can navigate in either direction.