The device was discovered in a building on the Henrik Ibsens Gate road.
Police were alerted to the object at around 10am on Tuesday. Shortly afterwards, the area was closed off and police bomb disposal teams arrived at the scene, broadcaster NRK reports.
The device was confirmed by police just before 12:30pm as being harmless.
Bombegruppen har undersøkt gjenstanden. Den viser seg å være ufarlig så sperringene oppheves og arbeidet kan gjenopptas. Vi drar straks fra stedet— OPS Politiet Oslo (@oslopolitiops) January 23, 2018
"It resembles a grenade-like object, but is not a typical grenade from the military," operation leader Christian Krohn Engeseth told NRK as police responded to the incident.
An image of the object is shown to photographers. Photo: Håkon Mosvold Larsen / NTB scanpix
The object was discovered by people working on a refurbishment in the building, the broadcaster reports.
It may have been there for a number of years, according to the report.
The commercial building is located near the Norwegian Foreign Ministry and the Royal Palace. The ministry was not evacuated.