The objective of this study was to evaluate changes in soil metal contents in an Ultisol cultivated with forage cactus (Nopalea cochenillifera) irrigated with diluted treated oil produced water (TOPW). The experiment was conducted in a randomized block design with five replications. The treatments consisted of 100% underground water (T1); 75% underground water plus 25% TOPW (T2); 50% underground water plus 50% TOPW (T3); 25% underground water plus 75% TOPW (T4); and 100% TOPW (T5). The plots consisted of 25 pots of 20 L in a greenhouse, in Mossoró, RN, Brazil. Soil samples from the 0-0.10 m and 0.10-0.20 m layers were collected at 240 days after planting, and Fe, Mn, Cu, Zn, Cr, Ni, Cd, and Pb contents were evaluated. The use of irrigations with diluted TOPW caused no contamination of the soils cultivated with forage cactus by the evaluated metals. T2 presented the lowest metal contents. Cr and Pb contents in the soil were significantly high, indicating that the application of TOPW should not be used continuously in the same area.