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optimistic expecting or tending to expect favorable outcomes. [2 definitions]
optimize to make the most effective use of (something), or to make as effective, perfect, or well-functioning as possible.
optimum the situation, quantity, degree, or other condition that is most desirable. [2 definitions]
option the right, power, or freedom to choose. [4 definitions]
optional not required; left to one's choice.
optoelectronic of or relating to electronic devices that are powered by light or produce light from electricity, or to the study of such devices.
optoelectronics electronic devices that are powered by light or that produce light from electricity. [2 definitions]
optometrist one who practices optometry.
optometry the profession of examining eyes, diagnosing defects in eyesight, and prescribing corrective lenses or exercises.
opulence the condition of being opulent. [2 definitions]
opulent having or displaying wealth or richness. [2 definitions]
opus a work of fine or literary art, esp. a musical composition numbered to indicate its chronological place in the composer's works.
-or1 a variant of -er1.
-or2 activity; quality; condition; characteristic.
OR1 abbreviation of "Oregon," a northwestern U.S. state on the Pacific coast between Washington and California.
OR2 abbreviation of "operating room."
or used to indicate alternatives. [3 definitions]
oracle in ancient Greece, a shrine, priest, or priestess dedicated to a god and often consulted for prophecies or advice. [3 definitions]
oracular of, like, pertaining to, or having the nature of an oracle; prophetic; wise; mysterious. [3 definitions]
oral spoken, as opposed to written; accomplished by speaking. [3 definitions]
oral history historical data, often tape-recorded, that comprises firsthand oral accounts and recollections relating to historical events or traditions. [2 definitions]