Managing Billing Address is shown first option in My Store Settings
Note: This manual is for osCommerce v4.
After you accessed the admin area of your website click on Settings, Configuration and My Store tabs. Then find Billing address is shown first option. If you click on Edit button you can make this option either true or false (in this way you can make the billing address show up first on checkout, so "same as" option will show up on the shipping address).