This is how the browser works by default unless you have enabled "Instant Search" in the Opera settings.
By default:
Opera for iOS resumes on the last open tab and restores the page if not loaded.
Instant Search enabled:
Opera for iOS resumes on last tab unless 2 hours has passed since last open. If so it will open a new tab and activate search automatically, to make it easy and quick to start searching or entering a url.
