Polymorphism and extensibility are features of OOP, but Haskell implements them in a rather different way to most OOP languages.

I realise people disagree about what exactly should be considered OOP, but personally I think not including inheritance in the definition broadens the term beyond usefulness.

There is no disagreement, Smalltalk is the object system. Object-orientation was invented in Smalltalk. Other languages and systems are just poor imitations.