I use olive oil to store feta cheese and black olives, each is in it's own container fully covered with oil then placed in the fridge for future use. my question is - why does the oil used with the olives turn solid (fatty), while the oil used with the feta cheese remains clear?
Each batch of olive oil has its own unique characteristics. See article on olive oil solidity: The oil in with the olives may be dissolving some of the waxes on the outside of the olives. The fact that the oil turns solid does not mean it is more or less fatty; olive oil is already 100% fat.