TheOsismii orOstimii were aGallic tribe living in the western part of theArmorican Peninsula (modernBrittany) during theIron Age and theRoman period.

They are mentioned asOsismi (var.ossismi) byCaesar (mid-1st c. BC),^[1]O̓sísmioi (Ὀσίσμιοι) byStrabo (early 1st c. AD),^[2]Ossismos byPliny (1st c. AD),^[3]O̓sismíous (Ὀσισμίους;var. Ὀσίσμιοι, Ὀσισμαίους) byPtolemy (2nd c. AD),^[4] and asOsismis in theNotitia Dignitatum (5th c. AD).^[5][6]

According to Strabo, theMassaliote explorerPytheas, who travelled to northwestern Europe in the late 4th century BC, reported the variantŌstimíous (Ὠστιμίους), which seems to be the earliest attested form of the name, documented before the Gaulishsound shift -st- > -ss- occurred.^[7]

TheGaulishethnonymOstim(i)i (sing.Ostim(i)os) literally means 'the ultimate', that is to say 'the remotest people', 'those who dwell at the extremity of theArmorican Peninsula'. It derives from theCeltic stemostim- ('ultimate, extreme'), itself from an earlier *postim- ('last'; cf. Lat.postumus 'last-born, final'). InMiddle French, the territory they occupied was known as (Fine) Posterne.^[7]

The territory of the Osismii was located at the extremity of theBrittany Peninsula, west of theVeneti andCoriosolites.^[8]

According to the consensus view summarised by Patrick Galliou, following Pierre Merlat and François Merlet, the territory of the Osismii extended across most of modernFinistère, parts of theCôtes-d'Armor (withTrégor and Goëlo regions), and slightly intoMorbihan. It was bounded roughly from theGouët estuary atSaint-Brieuc in the north to theEllé atQuimperlé in the south, a distribution corroborated by the spread of their pre-Roman coinage.^[9]

Their chief town was Vorgium (present-dayCarhaix), which theTabula Peutingeriana places at the tip of the Armorican peninsula. Ptolemy namesVorganium as the chief town of the Osismii. However, a milestone from northern Finistère suggests thatVorganium more likely refers instead to Kérilien (inPlounéventer), which was a different settlement in Gallo-Roman times and makes Ptolemy's identification probably incorrect.^[10]

Scholars often attribute the decline of Vorgium to a transfer of capital status. The creation of theTractus Armoricanus prompted the construction of coastal fortifications against Saxon piracy, shifting strategic priorities toward the coast and leading to the erection of a majorcastellum at Brest. In the early Middle Ages, Brest appears under the namesCivitas Osismorum andUrbs Osismi, probably derived from a 4th-centuryOsismis mentioned in theNotitia Dignitatum.^[10]

Some scholars identify theOestrymnici of Brittany, mentioned in theOra Maritima ofAvianus (4th century AD), with the Osismii. Avianus's poem preserves earlier geographical accounts derived from the voyage of the Carthaginian navigatorHimilco in the late 6th–early 5th century BC. If this connection is correct, it would place a people identifiable with the Osismii in Armorica already around 500 BC.^[11][12]

Here rises the head of a projecting ridge, which a more ancient age called Oestrymnis, and the lofty mass or rocky height completely faces the warm south wind Under the head of this promontory, the Oestrymnic bay lies open for the natives. In it the islands called Oestrymnides stretch themselves out. They lie widely apart and are rich in tin and lead. There is much hardiness in the people here, a proud spirit, an efficient industriousness. They are all constantly concerned with commerce. They ply the widely troubled sea and swell of monster-filled Ocean with skiffs of skin. For these men do not know how to fashion keels with pine or maple. They do not hollow out yachts, as the custom is, from fir trees. Rather they always marvellously fit out boats with joined skins and often run through the vast salt water on leather.

[...]

This place was first called Oestrymnis and the people inhabiting the area and the fields Oestrymnici. Afterwards numerous serpents put the inhabitants to flight and gave the evacuated land their name.

Around 320–300 BC, they are mentioned asŌstimíous (Ὠστιμίους) by the Greek explorerPytheas, whose account is reported byStrabo in the 1st century AD.^[14][11] Pytheas noted that the peninsula projected deep into the Ocean and ended at a headland calledKabaïon, identifiable with either thePoint Penmarc'h or thePointe du Raz. He also mentioned the nearby islands ofOuximasa, which is plausibly linked to CelticUxisama ('the highest') and often associated with the toponymUshant.^[14][11]

Secondly, there are the Osismii (whom Pytheas calls the Ostimii), who live on a promontory that projects quite far out into the ocean, though not so far as he and those who have trusted him say.

The Osismii submitted toCaesar during theGallic Wars in 58 BC.^[8]

At the same time he was informed by Publius Crassus, whom he had sent with one legion to the Veneti, Unelli, Osismi, Curiosolites, Essuvii, Aulerci, Redones—coastal communities that border the Ocean—that all those communities had been brought into the control and power of the Roman people

However, they took part in an insurrection against Rome in 56 BC.^[8]

Adopting this plan, [the Veneti] fortified their towns, brought provisions from the farmland into the towns, and collected as many boats as they could in the territory of the Veneti, where everyone agreed that Caesar would fight first. Their allies for that war were the Osismi, Lexovii, Namnetes, Ambiliati, Morini, Diablintes, Menapii. Auxiliary forces were summoned from Britain, which lies opposite that area.

In 52 BC, they sent troops to theBattle of Alesia.^[8]

A round ten thousand from all the communities that border the Ocean and customarily call themselves 'Armorican': the Curiosolites, Redones, Ambibarii, Caletes, Osismi, Veneti, †Lemovices†, and Unelli

• Veneti (Gaul)
• List of Celtic tribes

• Osismii
• Historical Celtic peoples
• Gauls
• Tribes in pre-Roman Gaul
• Tribes involved in the Gallic Wars
• Armorica

• Articles with short description
• Short description matches Wikidata
• CS1 errors: ISBN date