Ogmios (sometimes Ogmius; Ancient Greek: Ὄγμιος) is the name given to a Celtic god of eloquence described in Heracles, a c. 175 CE work of the Syrian satirist Lucian.

Lucian's Heracles is a prolalia, that is, a short text which was read aloud before a longer public performance. It describes Lucian's viewing of a strange image of Ogmios in Gaul, wherein the god is depicted as a dark-skinned, aged version of the Greek hero Heracles, with a group of happy devotees tied by bejewelled chains to this god's tongue. Lucian describes a Celt who approaches him and explains these features, informing him that they reflect a native association of Ogmios with eloquence, which, the Celt explains, reaches its highest level in old age. Lucian uses this anecdote to prove to his audience that, in old age, he is still competent to deliver public performances.

The evidence outside of Lucian's text for the god Ogmios is quite limited. No image has been uncovered which comes close to that which Lucian describes. The only further evidence for the god which has been largely accepted are on two curse tablets from Brigantium (in Austria), which invoke Ogmios's name. Most scholars accept the existence of the god Ogmios, but a minority have expressed scepticism.

In medieval Irish mythology, the god Ogma was fabled as the inventor of the early Irish alphabet Ogham. Ogmios has frequently been connected with Ogma, but the nature of this connection has proven difficult to define. An etymology linking Ogmios, Ogma, and Ogham poses unresolved chronological and phonological problems.

Lucian's text was much read in the Renaissance, and "Gallic Hercules" (as Ogmios was known) inspired a number of artistic works, including drawings by Albrecht Dürer and Hans Holbein the Younger.

Georges Dottin, Christian-Joseph Guyonvarc'h and Françoise Le Roux [fr] have proposed to derive the god's name derives from Greek ὄγμος (ógmos, "furrow, path"). Though Lucian tell us that Ogmios is the name of the god "in their native tongue", Guyonvarc'h and Le Roux believe it is possible the name may have been borrowed by Gaulish speakers from Greek in the parts of Gaul where Greek was widely spoken (such as Massalia). Jan de Vries is sceptical of this possibility. The Greek word ὄγμος seems to have had a connotation of leadership, which agrees with the iconography Lucian describes. The Nomina im Indogermanischen Lexikon notes the similarity with ἐπόγμιος (epógmios, "ruling over the furrows"), an epithet of the Greek god Demeter.

Celtic etymologies of the theonym have also been given. The potential existence of a reflex of the god's name in Irish mythology (Ogma, discussed below) has been taken to count in favour of such an etymology. Xavier Delamarre suggests that Ogmios is a reflex (through proto-Celtic) of proto-Indo-European *h₂óǵmos ("way"), derived from the PIE verbal root *h₂eǵ- ("to drive"). He associates with theonym with the meaning of "a leader along a path". Pierre-Yves Lambert suggested that Ogmios was a reflex of the proto-Celtic *oug- ("to sew").

Lucian (125 CE – after 180 CE) was a Syrian satirist and rhetorician who wrote in Ancient Greek. His short work Heracles or Hercules (Ancient Greek: Ἡρακλέα, romanized: Irakléa) is a prolaliai, that is, a short introduction intended to arouse audience interest prior to a longer lecture. It reflects on its author's old age, and his ability to deliver public oratory, concluding with an emphatic affirmation of this ability. On this basis, the text is dated late in Lucian's life, after his return from Egypt in 175 CE. The passage relevant to Ogmios comes at the beginning, where Lucian delivers an ekphrasis (literary description of a work of art) of an image of Heracles: