The Osismii, Ossismii, or Ostimii (also Ossismi, Osismi) were a Gallic tribe dwelling in the western part of the Armorican Peninsula (modern Brittany) during the Iron Age and the Roman period.
[5][6] According to Strabo, the Massaliote explorer Pytheas, who travelled to northwestern Europe in the late 4th century BC, reported the variant Ōstimíous (Ὠστιμίους), which seems to be the earliest attested form of the name, documented before the Gaulish sound shift -st- > -ss- occurred.
[7] 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 Gaulish ethnonym Ostim(i)i (sing.
Ostim(i)os) literally means 'the ultimate', that is to say 'the remotest people', 'those who dwell at the extremity of the Armorican Peninsula'.
This could be related to large peninsula of Cap-Sizun, on the southern-western coast of Finistère, just before reaching the difficult maritime pass of Pointe du Raz where the waters of the Atlantic Ocean and of the English channel are joining; this peninsula offers to the south a natural safe harbour in the aber and bay of today's town of Audierne, as well as in the bay of today's Douarnenez, to the north of the peninsula.