In Opencart, by default, after ordering, the user is redirected to the url /index.php?route=checkout/success. This again means that by default the code should be added to the file "/catalog/view/theme/yourthemedirectory/template/common/success.tpl".
If you are using a quick checkout page plugin on your opencart site, post-order users may be redirected to a url such as /index.php?route=quickcheckout/success. In this case, if the order is usually redirected to the thank you page with the same success file, but it redirects to a special page created by the plugin, you need to install the google ads conversion tracking code on the relevant page.