In 1971, Abu Osman Chowdhury was posted in Chuadanga, under district Kushtia as a Major of the Pakistan army and the Commander of 4th Wing East Pakistan Rifles. He left for Kushtia with his family on 23 March 1971 to attend an official meeting. He was staying at Kushtia Circuit Houseon the night of 25–26 March when the news of Operation Searchlight reached him. Sensing imminent danger he left Kushtia on early morning of 26 March 1971 and headed to Chuadanga via Jhenaidah while local political workers have already revolted. LaterBengali soldiers and raised the flag of Bangladesh at EPR 4 Wing Headquarter in Chuadanga. Later the 4 wing of EPR, led by Major Chowdhury and reinforced with Police and Ansar personnels and local youth, attacked 27 Baluch of Pakistan Army stationed at Kushtia and eliminated almost 2 companies. In the first sector commander's conference in July, Major Abu Osman Chowdhury was appointed the commander of the western sector, which comprised Kushtia, Jessore, areas of Faridpur, including Doulatpur-Satkhira Road encompassed within Khulna. It was past noon of 26 March when Major Abu Osman Chowdhury reached his Wing headquarters at Chuadanga. There, his NCOs briefed him of the overall situation including formal organisation of local resistance in the wake of the crackdown at Dhaka. In the meantime local Awami League leader Dr Ashab Ul Haq, who had earlier declared war against the occupational Pakistan armed forces the same morning at a public meeting, had contacted him over telephone and invited him to an emergency meeting with the public leaders and representatives of the local administration. At the meeting Major Osman was asked to take charge of the armed resistance force that he accepted at once. After a long discussion the first ever war command of Bangladesh, named South Western Command, was formed on 26 March 1971 in Chuadanga. While Major Abu Osman Chowdhury was given the position of the Commander, Dr Ashab Ul Haq, MPA became the Chief Advisor and Barrister Abu Ahmed Afzalur Rashid alias Badal Rashid, MNA, and Advocate Yunus Ali, MPA, were made Deputy Chief Advisors. The whole of western region of the Padma was taken under the command comprising that of Kushtia, Faridpur, Jessore, Khulna districts. All the armed personnel from defence, EPR, Police, Ansar, Mujahid and armed student wing of the area were vested under the Command. The newly built District Council Dak Bungalow was made the Command Headquarters. The next day on 27 March at about noon the Pakistani flag, last flying one at the EPR Wing headquarters was ceremoniously lowered and the tri-colour Bangladesh flag was hoisted at the flag post by Major Osman. Captain A R Azam Chowdhury, his deputy, was there who afterwards played valiant role under the Command. Major Osman held the position till division of Bangladesh war commands into 11 Sectors by the Provisional Government on 11 July 1971. The South Western Command was then renamed Sector-8 with some revision of the command area and Major Osman continued to hold the position of the Sector Commander till Major M Abul Manjur took over in September 1971. On 7 November morning, during the 1975 coup/soldier mutiny a group of renegade soldiers went to Abu Osman Chowdhuy's residence at Gulshan to kill him. Unable to find him, they murdered his wife. He retired from Bangladesh Army in 1976.
