Some long words that begin with the letter "O" include "obfuscate," which means to darken or confuse, and "obambulate," which means to wander around. Some other long words beginning with the letter "O" are "oblivescence," meaning forgetfulness, and "oneirocriticism," meaning dream interpretation.
Several long words beginning with "O" relate to ophthalmology, the branch of medicine concerned with the study and treatment of disorders and diseases of the eye. Some of these words include "ophthalmometer," a device to measure the eye, and "ophthalmophobia," the fear of being stared at.
Some long words beginning with "O" begin with "onomato-" or "onoma-," which are Greek roots related to names and words. "Onomatomania" is the irresistable desire to repeat certain words, while "onomastics" is the study of proper names.
Other long words beginning with "O" start with the prefix "ornith-," which relates to birds: "ornithology" is the study of birds, "ornithomancy" is divination by observing the flights of birds, and "ornithophobia" is the fear of birds.
Many other long or unusual words begin with "O," such as "orobathymetric," which relates to maps showing submerged heights and sea depths; "outrecuidance," which is overwhelming arrogance or self-esteem; and "oxyacaesthesia," which is an extreme sharpness of senses.