Is there an Orthodox Buddhism? Orthodoxy in Christianity is the holding to the oldest beliefs.  It is of course impossible for the religion not to evolve at all as new questions and there answer have to addressed. The "Orthodox" version is often considered to be the least evolved sect of a religion.  Does the concept of Orthodox apply in Buddhism?
Well, Theravada is often argued to hew most closely to the original teachings of the Buddha. See, e.g. "Two Main Schools of Buddhism" for a very brief overview of this. 

As I understand it, this has much to do with the fact that Theravada is the oldest of the extant Buddhist schools of thought, predating Mahayana by a few centuries. Theravada practice is also more localized to geographic regions nearer the birthplace of Buddhism (that is, Sri Lanka and Southeast Asia), and never strayed far from it; this is thought to have contributed to the "orthodoxy" of the school - it mixed less with other intellectual and spiritual traditions.

Additionally, Theravada Buddhists draw primarily on the Pali Canon (the Tripitaka), whereas Mahayana Buddhists draw primarily on Chinese sources. These Chinese sources are relatively more distant from the original teachings of the Buddha in terms of both time and language. As a byproduct of this, the Pali texts are often more "authentic".1

That said, the term "Orthodox" has no specific meaning in the context of Buddhism, i.e. there is no such school as "Orthodox Buddhism". There are only lowercase-o "orthodox schools of Buddhism", and what counts as "orthodox" will probably differ depending on who you ask. I should add that pre-sectarian Buddhism is almost definitionally the most "orthodox" school of Buddhist thought. The only problem is that, well, it doesn't exist anymore.



1 See, e.g. Rupert Gethin's The Foundations of Buddhism, p. 44: "it is likely that [it] is generally the case [...] that the traditions [the Pali recension] preserves are [...] the oldest and most authentic available to us" (elisions/etc because the original sentence was inverted)