Well, sgunhouse is right but let me still elaborate...

When you start Opera (29) there are 2 options. You can start in the Speed dial screen; this is basically an overview of a specific selection of bookmarks. Or you can choose to display one (or more) pages. But those pages will only be shown during startup, the "homepage" as you might know this on other browsers is basically the speed dial screen.

For example: if you close the last time you'll go back to speed dial.

Now, to set the starting page:

Go to your settings (alt-p) You'll see the "On startup" section in the "browser" tab

There you can pick what you'd like. Either Opera continues where you were, it opens the start page or you can make it open a specific page (or more).

Hope this helps!