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Oxyosteus is a genus of trout-sized, highly compressed arthrodire placoderms from the Late Devonian of Europe: The two described species are restricted to the Late Frasnian-aged Kellwasserkalk Fauna of Bad Wildungen, while a median dorsal plate of an unnamed species is known from the Middle Frasnian Holy Cross Mountains of Poland.

Species have, in cross section, an extremely compressed body, a pointed, highly elongated snout, and tremendous orbits. The living animals would have superficially resembled modern-day needlefish or houndfish. As per the family, the trunk shield is short. The genus is distinguished from other members of the family in that the plates are very thin, and the dermal surface is typically covered in small, closely packed tubercles. The genus was originally placed in its own family, but was then later determined to be closely related to Brachydeirus and Synauchenia, and was then merged into the family Brachydeiridae.