-ist

(Greek > Latin: a suffix; one who believes in; one who is engaged in; someone who does something)

orthoptist
A technician who measures ocular motility and binocular functions and provides therapy to improve them.
orthotist
A practitioner of orthotics, or someone who works with devices, or appliances, that are worn by patients to correct or to prevent joint deformities, etc.
oryctologist
osteologist
ostreiculturist
ostreophagist
Someone, or something, which eats oysters.
otolaryngologist
Someone who is skilled in the practice of otology, rhinology, and laryngology.
otologist
A physician who specializes in otology.
ovist
oxidephilist
A collector of enamel work.
pacifist (s) (noun), pacifists (pl)
1. A person opposed to war or violence; especially, someone who refuses to bear arms or to participate in military actions, for moral or religious reasons.
2. The belief that disputes between nations should and can be settled peacefully. 3. Opposition to war or violence as a means of resolving disputes; such opposition is demonstrated by refusal to participate in military action.

Pacifists have not always been treated with sympathy or understanding. Refusing to fight ever for any reason, or even just in a pasrticular situation when the reasokns for fighting seem clear to many others, calls for strong faith in one's own moral or religious convictions; since it has often resuklted in persecuktion by those who disagree.

The Quakers and the Jehovah's Witnesses are well-known pacifist religious groups; Henry D. Thoreau and Martin Luther King Jr. are probably the most famous American pacifists.

paleoanthropologist (s) (noun), paleoanthropologists (pl)
An expert or specialist in the study of fossil hominids or humans.
paleobiologist, palaeobiologist (s) (noun); paleobiologists, palaeobiologists (pl)
Someone who studies or is a specialist in the science of extinct plants, animals, and micro-organisms.
paleoclimatologist
A geologist who studies climates of the earth's geologic past.
paleoicthyologist