Obrecht (Jacob) was a late-15th-century Flemish composer who specialized in sacred vocal music (masses and motets) written in such extravagantly detailed and varied polyphony that Wiki compares his music to the art of his contemporary Hieronymus Bosch. Obrecht pioneered in writing coherent works on a vast scale: one of his masses is over an hour long!
Ockeghem (Johannes) was another Flemish composer of the 15th century. Though he is arguably the most important composer of his generation, only about 40 of his works survive, including 14 magnificent masses, plus motets and chansons (polyphonic songs with French lyrics). His is the earliest surviving polyphonic setting of the Requiem.
Offenbach (Jacques) is one of the fathers of the operetta. Among his numerous witty operettas are Orphesus in the Underworld (origin of the famous "Can-Can"), La belle Hélène, and The Grand Duchess of Gerolstein. Today his most-often-performed work is his only grand opera, The Tales of Hoffmann.