[SOLVED] Get Order items and WC_Order_Item

If you use the get_id() method, you get your item ID which is 15 in your code.

Get the product ID:
The correct WC_Order_Item_Product method to get the Product Id is: get_product_id()

Get the variation ID:
The correct WC_Order_Item_Product method to get the variation Id is: get_variation_id()

Get the order ID
The correct WC_Order_Item_Product method to get the Order Id is: get_order_id()

Get the WC_Product object
The correct WC_Order_Item_Product method to get WC_Product object is: get_product()

Get the WC_Order object
The correct WC_Order_Item_Product method to get WC_order object is: get_order()

Get and unprotecting the data and meta data using WC_Data methods:

get_data()
get_meta_data()

Get The WC_Product object from the order item ID:

$order_item_id = 15;
$item = new WC_Order_Item_Product($order_item_id);

// The product ID
$product_id = $item->get_product_id(); 

// The variation ID
$variation_id = $item->get_variation_id(); 

// The WC_Product object
$product = $item->get_product(); 

// The quantity
$quantity = $item->get_quantity(); 

// The order ID
$order_id = $item->get_order_id(); 

// The WC_Order object
$order = $item->get_order(); 

// The item ID
$item_id = $item->get_id(); // which is your $order_item_id

// The product name
$product_name = $item->get_name(); // … OR: $product->get_name();

// Get the product SKU (using WC_Product method)
$sku = $product->get_sku();

// Get line item totals (non discounted)
$total     = $item->get_subtotal(); // Total without tax (non discounted)
$total_tax = $item->get_subtotal_tax(); // Total tax (non discounted)

// Get line item totals (discounted when a coupon is applied)
$total     = $item->get_total(); // Total without tax (discounted)
$total_tax = $item->get_total_tax(); // Total tax (discounted)

Get the order items from the WC_Order object (and use the WC_product Object):

$order_id = 156; // The order_id

// get an instance of the WC_Order object
$order = wc_get_order( $order_id );

// The loop to get the order items which are WC_Order_Item_Product objects since WC 3+
foreach( $order->get_items() as $item_id => $item ){
    //Get the product ID
    $product_id = $item->get_product_id();

    //Get the variation ID
    $variation_id = $item->get_variation_id();

    //Get the WC_Product object
    $product = $item->get_product();

    // The quantity
    $quantity = $item->get_quantity();

    // The product name
    $product_name = $item->get_name(); // … OR: $product->get_name();

    //Get the product SKU (using WC_Product method)
    $sku = $product->get_sku();

    // Get line item totals (non discounted)
    $total     = $item->get_subtotal(); // Total without tax (non discounted)
    $total_tax = $item->get_subtotal_tax(); // Total tax (non discounted)
    
    // Get line item totals (discounted when a coupon is applied)
    $total     = $item->get_total(); // Total without tax (discounted)
    $total_tax = $item->get_total_tax(); // Total tax (discounted)
}

Accessing data and custom meta data:

1). Unprotecting WC_Order_Item_Product data and custom meta data:

You can use all WC_Order_Item_Product data methods or you can unprotect the data using WC_Data following methods:

$order_id = 156; // The order_id

// get an instance of the WC_Order object
$order = wc_get_order( $order_id );

// The loop to get the order items which are WC_Order_Item_Product objects since WC 3+
foreach( $order->get_items() as $item_id => $item ){

    // Get the common data in an array: 
    $item_product_data_array = $item->get_data();

    // Get the special meta data in an array: 
    $item_product_meta_data_array = $item->get_meta_data();

    // Get the specific meta data from a meta_key: 
    $meta_value = $item->get_meta( 'custom_meta_key', true );

    // Get all additional meta data (formatted in an unprotected array)
    $formatted_meta_data = $item->get_formatted_meta_data( ' ', true );


    // Get line item totals (non discounted)
    $total     = $item->get_subtotal(); // Total without tax (non discounted)
    $total_tax = $item->get_subtotal_tax(); // Total tax (non discounted)
    
    // Get line item totals (discounted when a coupon is applied)
    $total     = $item->get_total(); // Total without tax (discounted)
    $total_tax = $item->get_total_tax(); // Total tax (discounted)
}

2). The Array Access is still possible (for backwards compatibility with legacy arrays) to get the common data directly:

$order_id = 156; // The order_id

// get an instance of the WC_Order object
$order = wc_get_order( $order_id );

// The loop to get the order items which are WC_Order_Item_Product objects since WC 3+
foreach( $order->get_items() as $item_id => $item ){

    
    $product_id    = $item['product_id']; // Get the product ID
    $variation_id  = $item['variation_id']; // Get the variation ID

    $product_name  = $item['name']; // The product name
    $item_qty      = $item['quantity']; // The quantity

    // Get line item totals (non discounted)
    $line_total     = $item['subtotal']; // or $item['line_subtotal'] -- The line item non discounted total
    $line_total_tax = $item['subtotal_tax']; // or $item['line_subtotal_tax'] -- The line item non discounted tax total

    // Get line item totals (discounted)
    $line_total2     = $item['total']; // or $item['line_total'] -- The line item non discounted total
    $line_total_tax2 = $item['total_tax']; // The line item non discounted tax total

    // And so on ……
}

As reference:

Get Order items and WC_Order_Item_Product in WooCommerce 3

Accessing data and custom meta data: