Oil spillage and leakage of organic solvents have caused severe environmental and ecological damages. It is of great significance to develop a cost-efficient and green adsorbent material with high uptake efficiency to separate the oil-water mixture. In this work, biomass-derived CNOs were first time explored in the adsorption of organic pollutants and oils from water. Carbon nano-onions (CNOs) with hydrophobicity and oleophilicity were cost-effectively synthesized in an energy efficient flame pyrolysis process using flaxseed oil as a carbon source. The as-synthesized CNOs without any further surface modification have shown high adsorption efficiency in removing organic solvents and oils from the oil-water mixture. The CNOs could adsorb diverse organic solvents such as pyridine (36.81 mg g), dichloromethane (90.95 mg mg), aniline (76 mg mg), toluene (64 mg mg), chloroform (36.25 mg mg), methanol (49.25 mg mg), and ethanol (42.25 mg mg). The uptake capacity for petrol and diesel over CNOs was observed at 36.68 mg mg and 58.1 mg mg, respectively. The adsorption of pyridine followed pseudo-second-order kinetics and Langmuir's isotherm model. Moreover, the adsorption efficiency of CNOs towards the remediation of pyridine was almost similar in real-water samples when tested in tap water, dam water, groundwater, and lake water. Similarly, the practical applicability for the separation of petrol and diesel was also verified in the real sample (sea water) and has been proven to be excellent. By simple evaporation, the recovered CNOs can be reused for more than 5 cycles. CNOs exhibit the promising potential to be used in practical applications for oil-polluted water treatment.