I'm doubting this is a terrorist attack, hopefully. I heard it's a national holiday in Norway today so the area where the attack took place was pretty empty, which seems counter intuitive for terrorists. It might have been some construction work.. Hopefully not many are injured.
From the guardian
The explosion at around 3.30pm (2.30pm BST) blew out most windows on the 17-storey building housing prime minister Per Stoltenberg's office, as well as nearby ministries including the oil ministry, which was on fire.
Seems to be the perfect place to bomb