This study focused on the impact of used engine oil on the compressive strength of concrete. The compressive strengths of concrete cubes soaked in used mineral oil for 6 months were compared with those of oil-free concrete cubes. Five different concrete mixes were adopted to investigate the negative impact of mineral oil on low- and high-strength concrete. Furthermore, this paper describes an investigation into the effect of oil absorption on concrete frame foundations used for large machines at Assiut cement factory, CEMEX, in Egypt. It is concluded that the evaluation of the ggressive effect of mineral oil on concrete by experimentally comparing the strengths of oil-free concrete and oiled concrete at the same age is the most optimal and most reliable of the existing methods. The aggressiveness of oil is very clearly observed for low- and high-strength concrete, and the reductions in compressive strength were observed to be 17% and 11.8%, respectively. Additionally, based on this field study, the degree of oil absorption in concrete is an important determinant of compressive strength.