The rating reflects ONGC's position as the largest integrated oil and gas company in India with significant reserves.
A growing demand in India amid a global slowdown and government policies have made investments and returns attractive.
ONGC has only got one member on the board and virtually no say in the management.
The country buys over two-thirds of its oil, mostly from the Middle East, making it most vulnerable to any rise in oil price.
This investment is being made for drilling more than 220 oil and gas wells across the state, the company said.
The firm produced 24.23 million tonne of crude oil in the 2018-19 fiscal year and 25.81 billion cubic metres of natural gas.
Securities and Exchange Board of India had asked HPCL to re-file shareholding pattern to stock exchanges by August 13, 2019.
Oil and Natural Gas Corp in January last year bought the government's entire 51.11 pc stake in HPCL for Rs 36,915 crore.
Government is looking to raise up to Rs 10,000 crore through the follow-on fund offer of CPSE Exchange Traded Fund.
Union Cabinet in February had approved a policy for awarding acreage, based primarily on exploration work commitment.