Justice Thomas is a self-proclaimed Originalist who consistently votes with the conservative bloc of the Roberts Court, which has a reputation for being activist, so the appropriate answer to your question is: Both.
Explanation
"Originalist" and "activist" are two completely different concepts. The first refers to a particular approach toward interpreting the Constitution (trying to infer what the original Framers intended), while the second term refers to an inclination to make decisions without regard for precedent or legislation, in order to shape political or social policy from the bench. "Activism" tends to be used as a pejorative to describe progressive decisions, but in reality applies equally to conservatives. "Originalism" is fairly neutral, but also relatively meaningless as it connotes conservative ideology regardless of whether the ideology can be supported by the Framer's purported original intent.
Clarence Thomas considers himself an Originalist, but has allegedly never used historical citations to support his opinions, so the designation is difficult to support. People who refer to themselves as "Originalists" are typically conservative and appear to use the term to imply their decisions are more valid than those of justices who take more of a "living Constitution" approach to the law.
The conservatives on the Roberts' Court have made quite a few decisions that could have been considered "activist," insofar as they have declared certain Acts of Congress unconstitutional and overturned established precedents in order to advance policies that favor corporate interests and corporate personhood (difficult to defend as Originalist thinking). One of the more notable instances of conservative activism occurred in the decision of Citizens United v. Federal Election Commission, 558 U.S. 50 (2010), that opened the floodgates on corporate campaign spending and overturned legislation restricting "electioneering communication" (propaganda) immediately prior to an election.
Although we're taught the US Supreme Court is apolitical, in truth the Court has become increasingly partisan and divided by ideology. The Roberts Court has five conservative justices (although Kennedy is more moderate and sometimes votes with the progressive bloc) whose decisions tend to mirror the Republican agenda. Progressives view many of these decisions as activist, just as conservatives considered decisions of the liberal Warren Court activist.
Labels such as Originalism, Textualism, Strict Constructionism and so on, describing justices' interpretive approach to the Constitution are largely subjective and often meaningless. Each side harbors a particular general ideology, and many decisions are predictable on that basis alone. Each sides believes its decisions are constitutionally correct. Each side is adept at rationalizing its decisions using established precedents and constitutional provisions.
William O. Douglas: Majority and Dissent
Justice William O. Douglas supported this idea in his book, The Court Years 1939-1975: The Autobiography of William O. Douglas. In an anecdote about Charles Evans Whittaker, who had been assigned to write the opinion of the Court in the case Meyer v. United States, 362 US 410 (1960), Douglas explained how he came to write both the majority and dissenting opinion for the case:
"In one case when the vote was five to four, Whittaker was assigned to write the opinion for the majority. I had already written the dissent and went to his office to discuss a wholly different matter. When I entered he was pacing his office, walking around his desk with pursed lips as if possessed. I asked him what was wrong. He said, referring to the five-to-four decision, that he had been trying to write the majority opinion but simply could not do it.
'That's because you're on the wrong side,' I said.
'Not at all. Not at all. I am right but I can't get started.'
'Would you like me to send you a draft of the majority opinion?'
'Would you, please?'
"Within an hour the draft was in his office, and when the opinion came down it was one of the few in which the majority and minority opinions were written by the same man."
Douglas, William O. The Court Years 1939-1975: The Autobiography of William O. Douglas. New York: Random House, 1980.
