Six of the nine people have been rescued so far, and efforts are underway to save the remaining three, according to a company official.
The helicopter, which carried six ONGC employees and one contractor working for the company, was forced to land using floaters, which are attached to choppers that transport personnel and material from shore to offshore installations.
The chopper was attempting to land at the rig, located some 50 nautical miles from the Mumbai coast, when the incident took place. About 1.5 km from the landing zone on the rig, the chopper fell into the sea, the official said.
The circumstances that led to the incident were not immediately clear. Other details too were awaited, he said.
ONGC has several rigs and installations in the Arabia Sea that are used to produce oil and gas from reservoirs lying below the seabed.
ONGC deployed vessels from nearby installations to pull out the personnel onboard the helicopter.
According to the official, a rescue boat from the rig Sagar Kiran, near which the helicopter made an emergency landing, rescued at least one person.
Four people were rescued by the ONGC vessel Malviya-16.
