The are different types of "suggestions/autocompletions":
Server Name Completion
If enabled, Opera tries to complete the typed in domain name (after pressing enter) by a prefix (per default
www.) and a TLD (e.g.
.net etc.). So if you type into the address bar
asdfg/ (the slash is important) you probably will be directed to
www.asdfg.com -- or if there's no DNS entry, Opera will try
Preferences -> Advanced -> Network -> Server Name Completion or via various
opera:config entried (search for
Addressbar Inline Completion
"Automatic completion of previously typed addresses in the address field" (Opera Help)
Addressbar Search Suggestions
While you are typing, Opera sends the query to a search engine (per default that's G**gle) to make suggestions what you possibly are looking for.
For those who never came across it, you can simply copy & paste these
opera:config... URL's into the address bar and you'll end up at the correct position in the configuration list. Don't forget to press save after changing some options!