Acids attack metals and the patinas that form on metals. Olive oil is anywhere from 55% to 85% Oleic Acid, a mild organic acid that, given enough time, will reduce any copper coin to a blue-green sludge.

In addition to Oleic Acid, here are a few other unwelcome ingredients:

Linoleic Acid
Linolenic Acid
Free Fatty Acids
Phenols
Peroxides
Monoacylglycerols
Diacylglycerols
Triacylglycerols (Palmitic Acid, Stearic Acid and Arachidic Acid)
Thiobarbituric Acid reactive substances
Pheophytin A
Polycyclic aromatic hydrocarbons
Ascorbic Acid
Hexanal
2-hexenal
Trans-2-hexenal
1-hexanol
2-hexanol
3-methylbutan
Aldehydes
Volatile alcohols (propanol, amyl alcohols, 2-hexenol and heptanol)
Esters
Ketones
Furans
Octanal
Nonanal
Hydroxytyrosol
Tyrosol
Caffeic Acid
Coumaric Acid
p-Hydroxybenzoic Acid
Pentanal
2-pentenal
2-heptenal

In addition to this mesmerizing cocktail of chemicals, olive oil also has about 760mg of sodium per tablespoon. This sodium comes in the form of corrosive salts which, over time, will also precipitate unwanted surface metal erosion.

All of this having been said, none of these chemicals are so harmful that they will visibly harm a coin in overnight use (if that is somehow found to be useful). It is the long-term soaks that are bound to cause more problems than help.

Material compiled from the Olive Oil Source.