protected void RadGrid1_ItemCommand(object sender, GridCommandEventArgs e)
{
    if (e.CommandName == "MyCommand")
    {
        // e.Item.ItemIndex = 0 // wrong index
        // e.CommandArgument = "9" // correct index

        // Correct ItemIndex
        int itemIndex = int.Parse(e.CommandArgument.ToString());

        // access item by the correct itemIndex
        var dataItem = (sender as RadGrid).Items[itemIndex];
        // get the data key value of the correct item
        var orderId = dataItem.GetDataKeyValue("OrderID");
    }
}