There were nine people on board when the helicopter crashed near ONGC's rig Sagar Kiran in Mumbai High.
"Four persons rescued by Navy chopper were brought to the base unconsciously and taken to the hospital. Unfortunately, they could not survive," ONGC said, adding an inquiry had been opened.
An ONGC spokesperson said the company's offshore exploration efforts were not impacted due to the incident. He said the deceased including three ONGC employees and a contract worker.
Oil Minister Hardeep Singh Puri said he was "personally deeply saddened" by the accident and the loss of four lives.
"My heartfelt condolences to the bereaved families for their irreparable loss. We stand with them in this hour of grief," the minister tweeted.