@orangehorizon @g00g00 There is another way to do it:

1. Open opera://about/, copy the path to the profile (Paths - Profile), and also make a note of the path to the installation root (Paths - Install), close the browser,
2. Open your file explorer, and navigate to the directory of your profile, copy the siteprefs.json file to the installationroot\operaversion\resources subfolder, eg. installationroot\32.0.1948.69\resources,
3. Rename the default_partner_content.json to eg. default_partner-old, and rename siteprefs.json to default_partner_content.json, and then set the read-only permission to this file (this is essential, to make it work).
   In case, that there's no the siteprefs.json file, you can use this one.

One important thing, that I forgot to mention :doh: - if the default_partner_content.json exists also in the profile directory, then delete it, because otherwise the above solution is not going to work :spock:

Thanks @thatism3, your method works. Tested on Opera 33, default search engines removed.