At about 3:30 p.m. CEST (about 9:30 a.m. EST) a bomb exploded near the office of the prime minister in central Oslo, damaging buildings, blowing out windows, and causing at least 8 injuries. Reporting from the scene 45 minutes later is still chaotic. I'd rather point you at a coherent narrative, but there doesn't seem to be one yet; here's some of the details I've found from people near the scene.
Twitter hashtag: #oslo. On the scene: @finansakrobat, @geirolav, @chaglen. On Google Plus: Alexander Nørstad.
News: Aftenposten (in Norwegian): Stor eksplosjon i Oslo sentrum; same headline, NRK (in Norwegian). BBC: Norway: Blast near prime minister's office in Oslo. The Norwegian state broadcaster is NRK which has this as the lead story, many photos.
Blogs: Rachel Maddow: Explosion at Norway prime minister's office with some links.
Live blog: Reuters live blog: Explosion rocks Oslo. Guardian live blog: Oslo explosion – live coverage. BBC: Oslo Explosion.
Photos: from 20 sec after explosion; damage to windows (Aftenposten); damage to buildings with crowd visible (Christian Aglen). Twitter search: Oslo photos.