Onnaman movie is all about Ravishanker (Mohanlal) who is a street-urchin comes under the patronage of Salim Bhai, a self-styled messiah of the masses and a don too. When Ravishankar grows up, he becomes an adviser to Salim Bhai. With the help of his gang of young friends, Ravishankar soon emerges as a leader of the poor and the oppressed. Naturally, the 'bad' guys led by Gulab Chand Sha, resent him and do everything possible to eliminate Ravishankar.
How Ravishankar combats the 'evil' forces with the help of his friends, his ladylove, the district collector Kamala and the ACP Vishnu.S.Pillai form the rest of the story.