He was pronounced dead by doctors around 9am after being kept on life support over the last few days,
Democratic League's General Secretary Saifuddin Moni told bdnews24.com.
Ahad was admitted to Shamarita Hospital in a critical condition on Oct 14. He was suffering from lung infection.
His wife, Professor Rashida Begum said the veteran politician had been ill since March. He spent a spell in the hospital from March to April, before being taken home after his condition improved. He was brought back after it deteriorated again.
Ahad was one of the key founders of East Pakistan Muslim Chhatra League formed on Jan 4, 1948.
He was also the founding General Secretary of the Ganatantrik Juba League and was the first person to be arrested in the Language Movement on Mar 11, 1948.
Ahad, during his life-long political career, also served as the Publicity Secretary and Organising Secretary of East Pakistan Awami League.
He joined the National Awami Party (NAP) of Maulana Abdul Hamid Khan Bhashani through the Kagmari Convention in 1957.
The veteran politician played key roles in the movements for speech and personal freedom, democracy and freedom of press in Bangladesh, and had to spend a large part of his life in prison.
He also had to face trial in a military tribunal in the 80s. The weekly Ittehad, which was edited by him, was banned at that time for raising public opinion against dictatorship.
Oli Ahad was given the highest state recognition, Swadhinota Padak, in 2004 for his contribution to Bangladesh's independence and the Liberation War.
He was born in Islampur village of Brahmanbaria's Sorail. His only daughter Barrister Nomin Farzana practices in the High Court.