A few days ago I got an email from someone asking me about adding the default customer comments in OneStepCheckout (www.onestepcheckout.com) as regular order comments. I thought I’d share this quick & simple hack with everyone, so here’s what you have to do:
Open app/code/local/Idev/OneStepCheckout/Helper/Data.php, and after

$observer->getEvent()->getOrder()->setOnestepcheckoutCustomercomment($orderComment);

add this line:
$observer->getEvent()->getOrder()->setState( Mage_Sales_Model_Order::STATE_NEW, true, $orderComment, false );

$observer->getEvent()->getOrder()->setState( 
    $observer->getEvent()->getOrder()->getStatus(), 
    true, 
    $orderComment, 
    false 
);

This will add the comments on the regular comments/statuses thread, as well as a customer comment. If you want to disable customer comments, just comment the original code.

Actually, this approach should also work with the out-of-the-box Magento one step checkout. Add an input on the last step, create a listener for

checkout_type_onepage_save_order

and use the same piece of code:

$observer->getEvent()->getOrder()->setState( $observer->getEvent()->getOrder()->getStatus(), true, $this->_getRequest()->getPost('order_comments'), false );

Note: Tested on Magento 1.4.1.0 & 1.4.1.1

Cristi Magento Magento, onestepcheckout