object permanence The realization that objects (including people) still exist when they can no longer be seen, touched, or heard.

objective thought Thinking that is not influenced by the thinker’s personal qualities but instead involves facts and numbers that are universally considered true and valid.

Oedipus complex The unconscious desire of young boys to replace their father and win their mother’s romantic love.

old-old Older adults (generally, those over age 75) who suffer from physical, mental, or social deficits.

oldest-old Elderly adults (generally, those over age 85) who are dependent on others for almost everything, requiring supportive services such as nursing homes and hospital stays.

operant conditioning The learning process by which a particular action is followed by something desired (which makes the person or animal more likely to repeat the action) or by something unwanted (which makes the action less likely to be repeated). (Also called instrumental conditioning.)

organ reserve The capacity of organs to allow the body to cope with stress, via extra, unused functioning ability.

osteoporosis A disease whose symptoms are low bone mass and deterioration of bone tissue, which lead to increasingly fragile bones and greater risk of fracture.

overimitation When a person imitates an action that is not a relevant part of the behavior to be learned. Overimitation is common among 2- to 6-year-olds when they imitate adult actions that are irrelevant and inefficient.

overregularization The application of rules of grammar even when exceptions occur, making the language seem more “regular” than it actually is.