I found the solution by myself.
To add any code to <head></head> to a specific page (this worked for order-confirmation.tpl) add:
{block name='hook_extra'}
{literal}
!!!ADD CODE HERE!!!
{/literal}
{/block}
The order-confirmation.tpl is located in (themes/YOUR_THEME/templates/checkout).
Don't forget to use the {block name='hook_extra'}{/block}.
Don't forget to use {literal} {/literal}.
You can place the entire code at the end of the file.