Order has lookup to Account,Contact,Contract,Quote and Order and when you set OWD of Order object as Controlled by Parent and there is no Master-Detail relationship on Order Object then who is the parent that is controlling the Order Object?
From the Order documentation (my emphasis):
Represents an order associated with a contract or an account.
So I'd say the parent is either the Contract or the Account, depending on how the Order is setup.
The docs also say that
AccountID is a required field and that the
The more standard docs have:
Title for the parent contract that distinguishes it from other contracts. (Read-only)
And from Orders Overview:
Depending on your company’s needs, your administrator might require orders to be added to a contract or allow them to be added directly to an account.
So, based on the documentation, I'd say the Account is the parent that would usually drive OWD. It is required on all Order records.
The only exception might be orgs where the Contract is configured as required. This would need to be tested experimentally, unless someone has direct experience with it.