Magento – Magento 2: how to change default order status for Cash On Delivery Payment

Best idea to use event/observer concept.

Fire an observer on event: payment_method_is_active.

Observer:

<?php
namespace [Vendorname]\[Modulename]]\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\Request\DataPersistorInterface;
use Magento\Framework\App\ObjectManager;

class ObserverforDisabledCod implements ObserverInterface
{
    /**
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return void
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $result = $observer->getEvent()->getResult();
        $method_instance = $observer->getEvent()->getMethodInstance();
        $quote = $observer->getEvent()->getQuote();

        if ($quote === null || $method_instance->getCode() != 'cashondelivery')
            return;

        /* Disable All payment gateway exclude Your payment Gateway */
        $items = $quote->getAllVisibleItems();
        foreach ($items as $eachitem) {
            if ($eachitem->getProductType() == 'downloadable') {
                $result->setData('is_available', false);
                break;
            }
        }

    }
}

first create observer into events.xml app\code\Adamsmage\Statuschange\etc\events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="sales_order_place_after">
    <observer name="vendor_sales_order_place_after" instance="Adamsmage\Statuschange\Observer\OrderObserver" />
</event>
</config>

second create Observer file at following location app\code\Adamsmage\Statuschange\Observer\OrderObserver.php

<?php
namespace Adamsmage\Statuschange\Observer;

use Magento\Framework\Event\ObserverInterface;

class OrderObserver implements ObserverInterface
{
public function execute(\Magento\Framework\Event\Observer $observer)
{
    $order = $observer->getEvent()->getOrder();
    // also if you want to check condition here like only offline payment method than order status update etc....
    $order->setState("pending")->setStatus("pending");
    $order->save(); 
}