Soil communities, the microorganisms, are the initial and primary recipients of hydrocarbons or any xenobiotics incidented on the soil. This study evaluated the biology of soil organisms, ex-situ; the hydrocarbonclastics and non-hydrocarbonclastics, following an exposure to spent engine oil. The result demonstrated that at low and moderate contaminations of 1.5 and 2.5% w/w, hydrocarbons reduced the microbial population from control, 1.28 x109 cfu, to 3.42 x108 and 3.06 x 108cfu on week one respectively. However, at increased contaminations of 3.5% w/w, hydrocarbons from large oil spills weaken microbial ability to degrade the added hydrocarbons as the prejudicial nature reduced the population to 1.62 x 108cfu on week one. Overall, across the weeks, there was insurgence of hydrocarbon-degrading organisms, the hydrocarbonclastics, which on the other hand induced a limitation in microbial diversity. The isolated hydrocarbonclastic microorganisms in the descending order of their biodegradation potentials are: Pseudomonas aeruginosa, Arthrobacter, Alealigena, Corynebacter, Flavo-bacterium, Archromobacter, Micrococcus, Norcardia and Myco-bacterium. Thus within the ecosystem they devised metabolic diversity with increased catabolic properties primarily aimed at biodegrading the xenobiotics; making them hydrocarbonclastic organisms.