Code for creating order comment in magento and display in backend

Add code in config.xml in module in etc folder(Path : app/local/namespace/modulename/etc)

<events>
            <checkout_type_onepage_save_order>
                <observers>
                    <onestepcheckout_save_order_comment>
                        <type>helper</type>
                        <class>DCKAP_Onestepcheckout_Helper_Data</class>
                        <method>setOrderCommentCompatible1</method>
                    </onestepcheckout_save_order_comment>
                </observers>
            </checkout_type_onepage_save_order>     
            <checkout_type_onepage_save_order_after>
                <observers>
                    <onestepcheckout_save_order_comment>
                        <type>helper</type>
                        <class>DCKAP_Onestepcheckout_Helper_Data</class>
                        <method>setOrderCommentCompatible2</method>
                    </onestepcheckout_save_order_comment>
                </observers>
            </checkout_type_onepage_save_order_after>     
            <sales_model_service_quote_submit_after>
                <observers>
                    <onestepcheckout_service_quote_submit_order_comment>
                        <type>helper</type>
                        <class>DCKAP_Onestepcheckout_Helper_Data</class>
                        <method>setOrderCommentCompatible2</method>
                    </onestepcheckout_service_quote_submit_order_comment>
                </observers>
            </sales_model_service_quote_submit_after>    
        </events>


Add code in helper (path : app/code/local/namespace/modulename/helper)



public function isOrderComment() {
       
        return Mage::getStoreConfig('onestepcheckout/general/comment');
    }

public function setOrderCommentCompatible1($observer) {

        if ($this->getCompatibility() != self::COMPATIBILITY_TYPE1) {
           return;
        }
        if ($this->isOrderComment()) {
            $orderComment = trim($this->_getRequest()->getPost('order_comment'));
            if ($orderComment != "") {
                $observer->getEvent()->getOrder()->addStatusHistoryComment($orderComment,true);
            } else {
                $observer->getEvent()->getOrder()->setOrderComment(Mage::getSingleton('checkout/session')->getOpcheckoutOrderComment());
                Mage::getSingleton('checkout/session')->unsOpcheckoutOrderComment();
            }
        }
    }


    public function setOrderCommentCompatible2($observer) {

        if ($this->getCompatibility() != self::COMPATIBILITY_TYPE2) {
         return;
        }
        if ($this->isOrderComment()) {

            $orderComment = trim($this->_getRequest()->getPost('order_comment'));
            if ($orderComment == "") {
                $orderComment = Mage::getSingleton('checkout/session')->getOpcheckoutOrderComment();
                Mage::getSingleton('checkout/session')->unsOpcheckoutOrderComment();
            }
            $order = $observer->getEvent()->getOrder();
            if ($order && ($orderComment != "")) {
                if ($order->getId()) {
                    $resource = Mage::getSingleton('core/resource');
                    $sales_order = $resource->getTableName('sales_order');
                    $resource->getConnection('core_write')->update($sales_order, array('order_comment' => $orderComment), array('entity_id = ?' => $order->getId()));
                }
            }
        }
    }
Add code in template html file

<?php 


echo "comment add".Mage::helper('onestepcheckout')->isOrderComment();
if(Mage::helper('onestepcheckout')->isOrderComment()):?>
                                    <div>
                    <div>
                                            <ol class="checkout-agreements-ol">
                                                <li>
                                                    <?php $_MAX_CHAR = Mage::getStoreConfig('onestepcheckout/order/opcheckout_order_comment_length'); ?>
                                                    <?php $_msg = ''; $flag = false;?>
                                                    <?php if($_MAX_CHAR != '' && $_MAX_CHAR != 0): ?>
                                                        <?php $flag = true;?>
                                                        <?php $_msg = $this->__('Maximum Character(s):'.$_MAX_CHAR);?>
                                                    <?php endif; ?>
                                                    <label for="opcheckout_order_comment"><?php echo $this->__('Your Comment for this Order. ')?></label>
                                                    <br/><textarea name="opcheckout-order-comment" id="opcheckout_order_comment" class="opcheckout_order_comment"></textarea>
                                                      <?php echo $_msg ? $_msg : ''?>
                                                      <span id="comment_count_left"></span>
                                                </li>
                                            </ol>
                                            <script type="text/javascript">
                                                        <?php if($flag): ?>
                                                            var MAX_CHAR = '<?php echo $_MAX_CHAR; ?>';
                                                            var msg = '<?php echo $this->__('Character Left: ')?>';
                                                            $('comment_count_left').innerHTML = msg+MAX_CHAR;
                                                        <?php endif; ?>
                                                $('opcheckout_order_comment').observe('keyup',function(){
                                                            <?php if($flag):?>
                                                                var len = $(this).value.length;
                                                                if(len>MAX_CHAR){                                                        
                                                                    $(this).value = $(this).value.substring(0, MAX_CHAR);                                                                
                                                                }
                                                                $('comment_count_left').innerHTML = msg+(MAX_CHAR - $(this).value.length);
                                                            <?php endif; ?>
                                                    $('hidden_opcheckout_order_comment').value = $(this).value;
                                                });
                                            </script>
                                    </div>
                    </div>
                                    <?php endif; ?>