The OQA has taken some heat after Kripke's (arguably overblown) cleaving of metaphysical and conceptual questions. For example, in Slaves of the Passions, Schroeder writes (p.72):
Most discussions of whether any reductive view about any normative property could possibly be true focus on one or another version of Moore's Open Question argument. But this is hard to take seriously, once we clarify that reduction is a metaphysical thesis, rather than one about our normative concepts... Open Question arguments employ tests of cognitive significance--for example, they ask whether someone can believe that Ronnie has a reason without believing anything about Ronnie's desires, or conversely.... [T]his is a good test for whether the concepts are identical. But as I view reduction, it is not a view about our thoughts.... All of the Open Question tests for cognitive significance distinguish between Hesperus and Phosphorus... But no one concludes that Hesperus is not Phosphorus.
I used to be sympathetic to such dismissals of the Open Question argument, but now think them overly hasty. Standard counterexamples involve rigid designators whose associated (reference-fixing) sense clearly opens the possibility of their co-referring with some other (non-synonymous) term. 'Hesperus' refers to that celestial body we see in the evening, 'Phosphorus' to that celestial body we see in the morning, and it's clearly an open possibility that the celestial body we pointed to in the morning is one and the same as that which we pointed to in the evening.
No such story seems available in the case of normative language. The concept of goodness, for example, does not have an associated sense that makes clear how it might end up referring to a merely natural property. It isn't a 'gappy' concept (as Parfit puts it): it does not mean that natural property, whatever it is, that fills such-and-such a role.
So here's my argument against dismissing the OQA:
First, for ease of exposition, let's introduce some terminology. Say that a term like 'Hesperus' refers opaquely insofar as the sense of the term does not by itself settle its intension (reference across possible worlds). In order to know how to apply the term to imagined possible worlds, we first need to learn more about how the actual world is. (I take it that not all terms are like this. Descriptive terms, like 'red', are apt to be transparent, such that grasping the concept is sufficient to know - a priori - how to apply it to counterfactual worlds.) We can now claim:
(1) Only opaque terms are ineligible/unfit to feature in Open Question Arguments.
(2) The Bridge Principle: Whenever a term is opaque, there is a transparently associated (reference-fixing) functional property that serves as a 'bridge' between the sense and reference of the term. (See above examples of the properties associated with 'Hesperus' and 'Phosphorus'.)
(3) Conceptual analysis reveals that there is no such functional property transparently associated with normative concepts.
Hence, (4) Normative terms do not refer opaquely, in the manner of 'Hesperus' and 'Phosphorus'. Grasping the sense of the term (i.e. the concept) enables one to likewise grasp its referent.
Hence (5) Normative terms are eligible candidates to feature in successful Open Question Arguments.