First-quarter revenue fell by about 41% to 889.37 billion rupees.
Lockdowns around the world have knocked energy demand, forcing refiners to reduce crude processing and narrowed their refining margins.
"The company's sales during the month of April 2020 was impacted significantly by the nationwide lockdown and consequently capacity utilisation of the plants was lower," it said in a statement.
Refinery runs, however, returned close to normal levels by June, it added.
The company said its gross refining margin - profit from converting a barrel of oil into refined products - of minus $1.98 per barrel in the three months to June, compared with $4.69 per barrel a year earlier.
Refining margins have fallen globally due to low prices of gasoline, diesel and other petroleum products.
Asia April-June refining margins hit multi-year lows on COVID-19 shock
In the April-June quarter IOC operated its nine directly owned refineries at about 75% capacity.
Its fuel sales including exports fell 27% to 16.50 million tonnes.
IOC, along with its unit Chennai Petroleum, controls about a third of India's five million-barrels-per-day refining capacity.
By Nidhi Verma