It seems weird to me that Opera did not support adding search providers using the opensearch XML files. However, I found an alternative way to add search engines to Opera. Unfortunately, it requires the user to do it manually.
To add a search provider 1. Right click on the search field 2. Select ‘Create Search’ 3. Give the new search engine a title and a keyword.
It is very easy and smart, where the browser takes the post url and uses it as the search engine URL!