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Occupational therapist: Specially trained therapist who works with people who have disabilities to help them relearn how to perform daily activities.

Oncogene: A gene, one or more forms of which is associated with cancer. Many oncogenes are involved, directly or indirectly, in controlling the rate of cell growth. Oncogenes promote cell growth and multiplication. They are normally present in all cells but may undergo changes that activate them, causing cells to grow too quickly and form tumors.

Oncogenic: Giving rise to tumors, especially malignant tumors.

Oncologist: Doctor specializing in the diagnosis and treatment of cancer.

Oncology: Branch of medicine concerned with the diagnosis and treatment of cancer.

Oncology clinical nurse specialist: Registered nurse with a master's degree in oncology nursing who specializes in the care of cancer patients. Oncology nurse specialists may prepare and administer treatments, monitor patients, prescribe and provide care, and teach and counsel patients and families.

Oncology social worker: A person with a master's degree in social work who is expert in coordinating and providing non-medical care to cancer patients. They provide counseling and assistance to people with cancer as well as to their families, especially in dealing with the non-medical issues that can result from cancer, such as financial problems, housing (when treatments must be taken at a facility away from home), and child care.

Oncolysis: Dissolution or absorption of tumor cells.

Oncosis: A swelling or tumor, onset of tumor development.

Oophorectomy: Surgical removal of the ovaries.

Ophthalmologist: Medical doctor specializing in diagnosis and treatment of eye diseases.

Oral and maxillofacial surgeon: Doctor specializing in surgery of the mouth, jaw, and face.

Orchiectomy: Surgical removal of the testicles; castration.

Orthopedic surgeon: Doctor specializing in diseases and injuries of the bones.

Ostomy: General term meaning "opening", especially one made by surgery. (See: Colostomy, Ileostomy, Urostomy, and Tracheostomy.)

Otolaryngologist: Doctor specializing in diseases of the ear, nose, and throat. Also called a head and neck surgeon.

Ovary: Female reproductive organ located in the pelvic area. Normally a woman has two ovaries. They contain the eggs (ova) that, when joined with sperm, result in pregnancy. Ovaries are also the primary source of estrogen. (See: Estrogen.)

Overlapping clones: (See: Genomic library.)