Even Al qaida were caught on the wrong foot. From today's Guardian.
Terrorism analyst Will McCants told the New York Times that a group calling itself Ansar al-Jihad al-Alami, or the Helpers of the Global Jihad, issued a statement claiming responsibility for the attack, saying it was a response to Norwegian forces' presence in Afghanistan and to insults to the prophet Muhammad – a claim later withdrawn.
This shows how fragmented terrorist groups are today. It's not some top down group organising things, but a bunch of dispate autonomous organisations with a common agenda. The fact that they were ready to claim credit for something someone else did, shows how clueless their leadership is.
One of the reasons there were so many casualties is because Oslo didn't consider itself a target. It was one bomb that caused all this, and it was able to get very close to public buildings. In London there are bollards and other security features designed to stop this. The 7/7 attacks were lots of little bombs coordinated to go off together.