OBBLIGATO
(Italian) Literally, "necessary." Generally, a required, independent melodic part in a composition, usually soloistic, and secondary to the main melody of the work.
OFFEN
(German) Open.
OHNE
(German) Without.
OKTAVE
(German) Octave.
OSSIA
(Italian) Indicates an alternate part or fingering of a passage.
OUVERT
(French) Open; free.