problem.go

package main

import (
	"encoding/xml"
	"fmt"
)

const data = `
<?xml version="1.0" encoding="UTF-8"?>
<liverailapi requested="/order/line/list" api_version="4.11.1">
  <lines>
    <line>
      <order_id>123</order_id>
    </line>
    <line>
      <order_id>456</order_id>
    </line>
  </lines>
</liverailapi>
`

type OrderLinesResponse struct {
	XMLName xml.Name `xml:"liverailapi"`

	OrderLines []OrderLine `xml:"lines>line"`
}

type OrderLine struct {
	OrderId int `xml:"order_id"`
}

func main() {
	var v OrderLinesResponse

	if err := xml.Unmarshal([]byte(data), &v); err != nil {
		panic(err)
	}

	for _, o := range v.OrderLines {
		fmt.Printf("%p: %#v\n", &o, o)
	}
}