I don't know whether OSPRI definitively rebuts PolitiFact. The nature of the material suggests that authoritatively refereeing the debate would require more time than I am willing to devote to the project (and might perhaps exceed my ability).
There is one part of the debate, however, that is quite clear-cut. That portion occurs clearly when the PolitiFact editor responds to an op-ed that summarizes the case made in OSPRI's rebuttal (OSPRI's Bill Felker wrote the op-ed):
PolitiFact editor Tim Murphy replies: The OSPRI report prominently stated that “The most significant driver of out-migration [from Rhode Island] is the estate tax.” Our ruling, online at ProJoPolitiFact.com, was based largely on OSPRI’s own data, along with assessments from other experts. It was given a “false” rating because, in PolitiFact Rhode Island’s view, the report did not prove the case.PolitiFact Rhode Island relied on PolitiFact's "Burden of Proof" criterion in making its "False" ruling. As I have repeatedly emphasized, PolitiFact's use of its burden of proof criterion serves as an example of the fallacy of the appeal to ignorance (PolitiFact doesn't know x to be true, therefore x is false).
Illustrating the absurdity of PolitiFact's conclusion is simple. PolitiFact does not know x is false, or else no fallacious reasoning would be required to make the determination. Using the same reasoning PolitiFact uses to rate OSPRI's claim "False" we can likewise rate PolitiFact's claim (that OSPRI's claim is false) as "False."
The same claim cannot be false and not-false (~false) at the same time and in the same sense. That state of affairs is contradictory, and rightly regarded as an impossibility.
In terms of bias, this case concerning OSPRI simply constitutes another anecdote. PolitiFact's flawed system is not politically biased, just flawed. But where the flaw is applied unevenly, measurably affecting one ideology more than another, we have evidence of an ideological bias.