[1][4] After returning to Australia was employed as a senior lecturer at Monash University between 1967 and 1970.
[1][2][5] He received a Doctor of Science from Monash University in 1978, for his contributions to understanding the physiological anatomy of plant cells and tissues.
[6] From July 1986 to June 1987, O'Brien was elected president of the Royal Society of Victoria.
[2][8] He eventually moved back to his alma mater of Harvard University before retiring.
[2] O'Brien specialised in the study of the development and structure of wheat, and plant morphology more broadly.