Cholesterol produced by the liver is distributed throughout the body by lipoproteins. Lipoproteins are divided into low density (LDL) and high density (HDL). LDLs, which carry useful cholesterol into the cells, can narrow the veins over time and even clog, in which case the HDLs function and balance out the veins by purifying them from excessive cholesterol, which means bad. That's why LDL and HDL complement each other. If the body is rich in HDL, the unwanted cholesterol is easier to be removed.
Unsaturated fats increase the levels of cholesterol in the bloodstream, but polyunsaturated acids included abundantly in the vegetable oils decrease both levels of LDL and HDL in the blood. So, the balance between them is preserved. However, the oleic acid in olive oil contains abundant amounts of monounsaturated acids, which not only increases the LDL levels, more importantly, but also controls the HD levels that is effective against the harmful cholesterol. In other words, it reduces the 'bad cholesterol’ in the blood, it feeds the good cholesterol and increases the levels in the blood.