Oscan is an extinct Indo-European language of southern Italy. The language is in the Osco-Umbrian or Sabellic branch of the Italic languages. Oscan is therefore a close relative of Umbrian and South Picene.

Oscan was spoken by a number of tribes, including the Samnites, the Lucani, the Aurunci (Ausones), and the Sidicini. The latter two tribes were often grouped under the name "Osci". The Oscan group is part of the Osco-Umbrian or Sabellic family, and includes the Oscan language and three variants (Hernican, Marrucinian and Paelignian) known only from inscriptions left by the Hernici, Marrucini and Paeligni, minor tribes of eastern central Italy. Adapted from the Etruscan alphabet, the Central Oscan alphabet was used to write Oscan in Campania and surrounding territories from the 5th century BCE until at least the 1st century CE.

Oscan is known from inscriptions dating as far back as the 5th century BCE. The most important Oscan inscriptions are the Tabula Bantina, the Oscan Tablet or Tabula Osca, and the Cippus Abellanus. In Apulia, there is evidence that ancient currency was inscribed in Oscan (dating to before 300 BCE) at Teanum Apulum. Oscan graffiti on the walls of Pompeii indicate its persistence in at least one urban environment well into the 1st century of the common era.

In total, as of 2017, there were 800 found Oscan texts, with a rapid expansion in recent decades. Oscan was written in various scripts depending on time period and location, including the "native" Oscan script, the South Oscan script which was based on Greek, and the ultimately prevailing Roman Oscan script.

In coastal zones of Southern Italy, Oscan is thought to have survived three centuries of bilingualism with Greek between 400 and 100 BCE, making it "an unusual case of stable societal bilingualism" wherein neither language became dominant or caused the death of the other; however, over the course of the Roman period, both Oscan and Greek were progressively effaced from Southern Italy, excepting the controversial possibility of Griko representing a continuation of ancient dialects of Greek. Oscan's usage declined following the Social War. Graffiti in towns across the Oscan speech area indicate it remained in colloquial usage. One piece of evidence that supports the colloquial usage of the language is the presence of Oscan graffiti on walls of Pompeii that were reconstructed after the earthquake of 62 CE, which must therefore have been written between 62 and 79 CE. Other scholars argue that this is not strong evidence for the survival of Oscan as an official language in the area, given the disappearance of public inscriptions in Oscan after Roman colonization. It is possible that both languages existed simultaneously under different conditions, in which Latin was given political, religious, and administrative importance while Oscan was considered a "low" language. This phenomenon is referred to as diglossia with bilingualism. Some Oscan graffiti exists from the 1st century CE, but it is rare to find evidence from Italy of Latin-speaking Roman citizens representing themselves as having non–Latin-speaking ancestors.

Oscan speakers came into close contact with the Latium population. Early Latin texts have been discovered near major Oscan settlements. For example, the Garigliano Bowl was found close to Minturnae, less than 40 kilometers from Capua, which was once a large Oscan settlement. Oscan had much in common with Latin, though there are also many striking differences, and many common word-groups in Latin were absent or represented by entirely different forms. For example, Latin volo, velle, volui, and other such forms from the Proto-Indo-European root *welh₁- ('to will') were represented by words derived from *ǵʰer- ('to desire'): Oscan herest ('(s)he shall want, (s)he shall desire', German cognate 'begehren', Dutch 'begeren', English cognate 'yearn') as opposed to Latin volent (id.). Latin locus (place) was absent and represented by the hapax slaagid (place), which Italian linguist Alberto Manco has linked to a surviving local toponym.

In phonology too, Oscan exhibited a number of clear differences from Latin: thus, Oscan 'p' in place of Latin 'qu' (Osc. pis, Lat. quis) (compare the similar P-Celtic/Q-Celtic cleavage in the Celtic languages); 'b' in place of Latin 'v'; medial 'f' in contrast to Latin 'b' or 'd' (Osc. mefiai, Lat. mediae).

Oscan is considered to be the most conservative of all the known Italic languages, and among attested Indo-European languages it is rivaled only by Greek in the retention of the inherited vowel system with the diphthongs intact.