O
Obbligato
Necessary. Opposite of ad lib
Offen (Ger)
Open
Ohne dämpfer (Ger)
Without mute, like senza sord

Opera
A musical drama where the words are sung, not spoken
Ophecleide
Brass instrument, French ancestor of the tuba
Orchestra
Large group of classical instrumentalists, traditionally comprising strings, woodwind, brass and percussion.
Ostinato
Repeated phrase, often in the bass as an accompaniment
P
Percussion family
Instruments that produce sounds of definite or indefinite pitch when shaken or struck, including drums, rattles, bells, gongs, and xylophones
Pesante
Heavy
Phrase
A relatively short portion of a melodic line which expresses a musical idea, comparable to a line or sentence in poetry
Piano
1 - keyboard instrument, whereby strings are struck by hammers. Can be classified as keyboard, percussion or strings!
2 - "soft" dynamic marking
Pizzicato
"Pinched." On string instruments, plucking the string
Poco
Little. Also poco a poco, little by little
Piu mosso
More motion
Prelude
"Play before." An introductory movement or piece