RFE/RL correspondent Kubanych Joldoshev reports the blast was on the third floor of a building that houses several independent newspapers.
Two of those newspapers are on the building's third floor.
There were no immediate reports of injuries.
Zamirbek Sydykov, spokesman for the Osh police, said the force of the explosion was [quite] powerful.
"That is why the walls on three sides of the lavatory [where the explosion occurred] were destroyed and smashed into the main wall. As a result, the neighboring rooms -- the journalists' rooms -- were damaged as well. All the windows are broken," Sydykov said.
Police have detained one women who had been acting suspiciously nearby.
