As a correction, it's wrong to describe OCaml's types as being polymorphic (I expected this to be polymorphic types in OCaml, which would have been far more interesting!)
All of the types used by OCaml are monomorphic. The expressions written in OCaml are polymorphic - they can have multiple types.
An expression like:
fn f => (f true) :: (f nil)
is untypeable in OCaml; f must have some type tau -> tau', applying it to true means that tau = bool, applying to nil means that tau = a list, and a list <> bool.
The expression itself can have many types; we choose the type we care about based on context.
Polymorphic types mean that one type can be many things; one could use them to (for example, if we allowed forall quantifiers) prove that function to be of type (forall alpha.alpha -> alpha) -> bool list.
This turns out to be undecidable for languages like OCaml, but simpler languages (usually ones that admit no nontermination) sometimes allow it.
