Did you actually close the browser, or did you close the tabs? If you click the close box (top right corner of the window) or select Exit from the menu, it really will close. When you then restart the program later, it will start however you've configured it in Settings (default is to reopen all the tab which were there when you clused it.)
If, on the other hand, you close all the tabs the browser itself does not close and shows a new tab for you. There are extensions which can change what page is in a new tab, but Opera has always been designed to not close the browser when you close all the tabs.