The combination denotes a diphthong, IPA [oe], that had a value similar to English "OI". It was used in borrowings from Greek words having the diphthong "OI" ("ΟΙ"). Both classical and modern practice is to write the letters separately, but the ligature was used in medieval and early modern writings, in part because "Œ" was reduced to a simple long vowel (IPA [e:]) in late Latin.
Borrowings into English from Latin words featuring "Œ" are often spelled "E", especially in American English. For example, foederal became federal in English, while foetid became fetid in American English.
For computers, when using the Unicode character set, the codes for 'Œ' and 'œ' are respectively 338 and 339, or 152 and 153 in hexadecimal.
In HTML, you can also use the HTML character entity references
Oe, Tokushima is a district of Japan.