Tweak Daddy

First, open Opera 10. There enter into the Tools option. Hit the preferences button and there go to the search tab. It will give options like add, delete and edit. First, select the Google search engine and click edit. Then, click on more details in the context box and copy the url that's shown.

After this go back to the search tab and click on add button, there type the search engine name and keyword you want to use. Click on more details button and you will get a space for typing the url. There paste the url you copied earlier and change at the beginning of the url from "www.google.com" to "www.bing.com" . Do not change any other part!! Your integrated search engine will be ready to use.

Note: If you just write the url as "www.bing.com" it won't work.

Here is a way to add Bing search in Google Chrome