Retrieve items on Order Magento 2

The correct way to load Order object (which will carry the items info in itself) is via Repository objects.

Inject the order repository in your constructor

protected $orderRepository;

public function __construct(
    ...
    \Magento\Sales\Api\OrderRepositoryInterface $orderRepository,
    ....
) {
    ....
    $this->orderRepository = $orderRepository;
    ....
}

and use in required method as:

$order = $this->orderRepository->get($orderId);
foreach ($order->getAllItems() as $item) {
// var_dump($item->getData());
}

To know more about Repository Objects: http://alanstorm.com/magento_2_understanding_object_repositories

$order = $this->_objectManager->create('Magento\Sales\Model\Order')->load($orderId);
$orderItems = $order->getAllItems();

You done it using objectManager

below i code which will help you to get order items (i called in Block class)

<?php
namespace Sugarcode\Test\Block;

class Test extends \Magento\Framework\View\Element\Template
{
    protected $order;

    public function __construct(
        \Magento\Backend\Block\Template\Context $context,
        \Magento\Sales\Model\Order $order,
        array $data = []
    ) {
        $this->order = $order;
        parent::__construct($context, $data);
    }



    public function _prepareLayout()
    {
        return parent::_prepareLayout();
    }
    public function getOrderItems()
    {
        $order=$this->order->load(1);

        return $order->getItems();

    }
}

if it works accept the answer which will helps others