The 250-kilogramme dud was discovered during construction work in a garden midday in the city centre on Wednesday, and experts decided it was too corroded and unstable for transport.
Some 10,000 people in the area were evacuated and the train station was closed for several hours, causing Deutsche Bahn to bus passengers into the city.
The bomb was detonated shortly after midnight and police reported no immediate damages to nearby structures. Residents were allowed to return to their homes before dawn.
In January, 15,000 Osnabrück residents were forced to evacuate while experts defused another WWII bomb.
More than 60 years after the end of World War II, weapons recovery remains an important task for police throughout Germany. Allied forces dropped more than 2.7 million tonnes of explosives across Germany during the war. Some of the ordnance did not explode and has become increasingly dangerous with time and corrosion.
Entire neighbourhoods are frequently evacuated for bomb removal, and most are safely defused. Construction and road workers are trained to call emergency services the moment they suspect they've found unexploded ordnance, but accidents still occasionally happen.
In 1994, three construction workers were killed and eight bystanders injured when an unexpected bomb detonated, tearing through nearby buildings and cars in Berlin. In 2006, a road worker was killed near Frankfurt when his excavator hit a bomb.