[Customizing checkout fields using actions and filters] Display WooCommerce custom checkout field admin order page.
/**
* Display field value on the order edit page
*/
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );
function my_custom_checkout_field_display_admin_order_meta($order){
echo '<p><strong>'.__('My Field').':</strong> ' . get_post_meta( $order->id, 'My Field', true ) . '</p>';
}