The rebels reportedly gunned down the workers as they returned from a drilling site some 290km south of Dispur, capital of the remote northeastern state.
Police said the attack was unrelated to a federal government warning earlier this month to oil companies - including ONGC - that they could be attacked by Islamic militants in Assam, including members of the infamous al Qaeda network.
"These were tribals who carried out the attack," said a senior police officer. "Six people, including a senior scientist, died on the spot and six others were seriously injured." Two died later in hospital.
Police said the attackers belonged to the Dima Halong Dagah tribal group who have been fighting for six years for more political autonomy for the area, accusing the government of neglecting economic development.
It was the first time the group had targeted oil workers in the state which produces more than five million tons of crude annually - about 15 percent of India's total onshore output.
