Œ (minuscule: œ) is a Latin alphabet grapheme, a ligature of o and e. In medieval and early modern Latin, it was used to represent the Greek diphthong οι and in a few non-Greek words, usages that continue in English and French. In French, it is also used in some non-learned words, representing then mid-front rounded vowel-sounds, rather than sounding the same as é or è, those being its traditional French values in the words borrowed from or via Latin.
It is used in the modern orthography for Old West Norse and is used in the International Phonetic Alphabet to represent the open-mid front rounded vowel. In English runology, œ ɶ is used to transliterate the Runic letter odal (Old English ēðel "estate, ancestral home"). Its traditional name in English is ethel or œthel. It is also sometimes used to translate the Anglo-Saxon letter Othala (ēðel, odal).
Classical Latin wrote the o and e separately (as has today again become the general practice), but the ligature was used by medieval and early modern writings, in part because the diphthongal sound had, by Late Latin, merged into the sound e. The classical diphthong had the value oe̯, similar to (standard) English oi as in choice. It occurs most often in borrowings from Greek, rendering that language's οι (= in majuscule: ΟΙ), although it is also used in some native words such as coepi.
In French, œ is called e dans l'o [ə dɑ̃ lo], which means e in the o (a mnemotechnic pun used first at school, sounding like (des) œufs dans l'eau, meaning eggs in water) or sometimes o et e collés, (literally o and e glued) and is a true linguistic ligature, not just a typographic one (like the fi or fl ligatures), reflecting etymology. It is most prominent in the words mœurs ("mores"), cœur ("heart"), sœur ("sister"), œuf ("egg"), bœuf ("beef", "steer"), œuvre ("work") and œil ("eye"), in which the digraph œu, like eu, represents the sound œ (in other cases, like plurals œufs ("eggs") and bœufs ("steers"), it stands for ø).
French also uses œ in direct borrowings from Latin and Greek.