Don't know how I missed it, but the code for change_email_confirm is located in oc-includes/osclass/controller/user-non-secure.php, line 35.
There is also a hook: osc_run_hook('change_email_confirm', Params::getParam('userId'), $userOldEmail, $userEmailTmp['s_new_email']);.
The hook also passes old email so there is no need for init code in the upper post.
In case anyone else will be in a similar situation, Windows Grep is a great tool for searching a lot of files for a specific string.
Regards.