In order to configure OpenCart to work properly with another domain, you should modify the two configure.php files in the OpenCartInstallationFolder/ and the OpenCartInstallationFolder/admin/ folders to reflect the new domain.

The following lines of the OpenCartInstallationFolder/configure.php file should be changed: 

// HTTP

define('HTTP_SERVER', 'http://yournewdomain.com/');

define('HTTP_IMAGE', 'http://yournewdomain.com/image/');

define('HTTP_ADMIN', 'http://yournewdomain.com/admin/');

// HTTPS

define('HTTPS_SERVER', 'http://yournewdomain.com/');

define('HTTPS_IMAGE', 'http://yournewdomain.com/image/');

Swap the yournewdomain.com string with your new domain. 

Perform the same modifications in your OpenCartInstallationFolder/admin/configure.php file:

// HTTP

define('HTTP_SERVER', 'http://yournewdomain.com/admin/');

define('HTTP_CATALOG', 'http://yournewdomain.com/');

define('HTTP_IMAGE', 'http://yournewdomain.com/image/');

// HTTPS

define('HTTPS_SERVER', 'http://yournewdomain.com/admin/');

define('HTTPS_IMAGE', 'http://yournewdomain.com/image/');