Overriding Helper

I’m trying to override the Helper_Data of a thirdparty module. But for some reason it wont work.

app/etc/modules/My_CustomModule.xml

<?xml version="1.0"?>
<config>
    <modules>
        <My_CustomModule>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Idev_OneStepCheckout/>
            </depends>
        </My_CustomModule>
    </modules>
</config>

.

app/code/local/My/CustomModule/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <My_CustomModule>
            <version>0.1.1</version>
        </My_CustomModule>
    </modules>
    <global>
        <helpers>
            <onestepcheckout>
                <rewrite>
                    <data>My_CustomModule_Helper_Data</data>
                </rewrite>
            </onestepcheckout>
        </helpers>
    </global>
</config>

.

app/code/local/My/CustomModule/Helper/Data.php

<?php
    class My_CustomModule_Helper_Data extends Idev_OneStepCheckout_Helper_Data
    {   
        public function setCustomerComment($observer)
        {
          Mage::log(get_class($this), null, 'test.log');
          // REST OF PARENT FUNCTION I WANT TO OVERRIDE
        }
    }

I already added a Mage::log to the function I’m trying to override but this keeps returning the classname of the original function.

UPDATE:

I did some digging in the code and found out that the function setCustomerComment is called by the observer event. I have no idea what this exactly does and or this just simply overrules the override?

app/code/local/Idev/OneStepCheckout/etc/config.xml

<?php    
        <events>
            <checkout_type_onepage_save_order>
                <observers>
                    <onestepcheckout_save_customercomment>
                        <type>model</type>
                        <class>Idev_OneStepCheckout_Helper_Data</class>
                        <method>setCustomerComment</method>
                    </onestepcheckout_save_customercomment>
                </observers>
            </checkout_type_onepage_save_order>
            ...
        </events>

I also created a file “test.php” in the root of magento with the code:

test.php

<?php

include "app/Mage.php";
Mage::app();

$customerHelper = Mage::helper('onestepcheckout/data');
var_dump(get_class($customerHelper));

This returns the module I have created, so it should override the initial class and functions right?

Any help would be appreciated.