The concentration of essential elements K, Ca and Mg and non-essential elements Cr, Cd and Pb were investigated in onion bulb and irrigation water used for cultivating onion at Ketar Kotebula Kebele (Golja) irrigation site in Tiyo Woreda, Arsi Zone, Oromia region, Ethiopia. A composite sample of irrigation water were taken from the two farm sites during 28-29/01/2019 and 23-24, and while a composites of Onion bulb sample were taken from two plots of each two vegetable farm sites during 23-24, May 2019. Dissolution of onion samples was carried out by wet digestion with a total volume of 2mL HNO3 (65%) and 2 mL HClO4 (70%) mixture on hot plate apparatus. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the levels of essential and non-essential heavy metals. The method validated by performing parameters such as instrumental calibration, spiking sample, method detection limit and precision of analytical procedures. The levels of major metals were K (20.05 ± 2.19 mg/L), Ca (12.84 ± 0.0216 mg/L) and Mg (3.71 ± 0.104 mg/L), where the levels of nonessential metals were Cr (0.01±0.001 mg/L), Pb (0.0036 ± 0.004 mg/L) and Cd (not detected) in irrigation water from Ketar River. The levels of major metals were K (4726 ± 0.025 mg/Kg), Ca (1730.56 ± 0.98 mg/Kg) and Mg (899.09 ± 0.99 mg/Kg), where the level of non-essential metals were Cr (1.367 ± 0.009 mg/Kg), Pb (4.1 ± 0.015 mg/Kg) and Cd (not detected) in onion irrigated with water from Ketar River. The results showed that Cr and Pb concentrations exceeded the safety limits given by Food and Agriculture Organization (FAO) or the World Health Organization (WHO) for human consumption with the exception of cadmium that was lower than the permissible leveling in all of the samples. It is recommended that, appropriate measures should be taken by population around the area and the government to minimize excess uses of agrochemicals and anthropogenic activities. Keywords: heavy metals, essential metals, irrigation, concentration