How do I chose a table to contain a foreign key for one-to-one relationship?

Are there some fundamental database considerations for selecting which
side of one-to-one relationship should store foreign key? This question
does not come up in one-to-many relationships but what are the
considerations in one-to-one case?

Let say for example that in my RoR application I have models for
purchase order and invoice. Do I select that Order has_one Invoice and
Invoice belongs_to Order or do I swap has_one/belongs_to usage? Both
options provide me with methods to access order from invoice and visa
versa.

Given the fact that placement of belongs_to corresponds to a table
which contains foreign key the equivalent question is how one decides
to put order_id foreign key into Invoices table or invoice_id into
Orders table? How much does it really matter which option I go with?