I don't think so

There isn't one as far as I know. What you can do while visiting a website full of ads is to right click on the page, you will see Opera drop down menu, choose "Block content" and then click on the images/adverts you want to block, you will see a red line appears saying "Blocked image" It is not the same as an ad blocker on the fly but better than nothing. BTW: I have just read about the next Opera10.5 (beta already out) and it seems it will include a privacy mode just as Firefox/Safari/IE have, this is good news!