I have two simple tables "items" and "orders". For the sake of simplicity lets assume that one item can only be in one order or one order can only contain one item.
Now as this can be implemented using simple one to one relationship I can do following:
I can add the primary key of the orders table to the items table like below
item_id, item_name, order_id
1, shoes, 1
2, watch, 2
order_id, customer, item_id
1, James, 1
2, Rick, 2
One-to-One relationships should be generally merged simply into one table. If there aren't any contradictions, One-to-One relationship might be a sign of an unconsidered decision.
And If You really want to use this kind of relationship, it's totally up to You where to place FK. You might want to take optionality into consideration when applying FK. However, in MySQL, it still won't be a true One-to-One relationship because deferred keys are not supported there.