Occlusal wear - loss of enamel¹ on the biting surface of the teeth due to the abrasive² action of chewing natural unprocessed food.

    Odontoblast process - the extension of the cy lasm of an odontoblast which remains³ surrounded by dentine during tooth formation. The process is still an active part of the cell and contributes to the production of intratubular dentine in response to ageing, tooth wear or arrested caries.

    Odontoblasts- cells lining⁴ the dental pulp⁵, derived⁶ from the dental papilla wich form the dentine of the tooth crown and root. New odontoblasts may become differentiated⁸ from less specialised pericytes in the pulp.

    Occlusal interference - contact between opposing teeth during chewing, which prevents the other teeth touching⁹. In extreme lateral¹⁰ and protrusive¹¹ positions of the jaw¹², this would happen in most dentitions and be of no concern. When occlusal interference occurs close to the area of maximum tooth contact, it may be troublesome.

    Oncogenes - genes¹⁴ which have the capacity if expressed to cause tumour¹⁵ formation.

    Organelle - a structures within a cell which has a specific structure or function, such as thenucleus, endoplasmic reticulum, golgi apparatus¹⁶, lysosomes and mitochondria.. Cell organelles are a feature of eucaryotic cells.

    Osmotic pressure - Water tends to move towards dense¹⁷ concentrations of ions. Sugar solutions on the surface of exposed dentine cause water to be drawn¹⁸ out of the dentinal tubules causing which distorts the odontoblast causing pain.

    Osseous integration¹⁹ - a term used to describe the desired adhesion between an implant²⁰ and the bone which holds it in place.

    Osteoblasts - cells which differentiate⁷ from pericytes and secrete²¹ both the matrix and mineral of bone.

    Osteocalcin - a calcium²² binding²³ protein, synthesised by the osteoblast and secreted²⁴ into the matrix at the time of bone mineralisation. Mice bread without the osteocalcin gene¹³ develop heavy bones suggesting that osteocalcin is a negative regulator of bone formation

    Osteoclast - a multi-nucleate cellcapable of removing both the organic and mineral component²⁵ of bone. Osteoclasts activity is controlled by nearby osteoblasts.

    Osteoid - the extracellular matrix inwhich bone forms. It is high in collagen and other bone proteins but lacks any crystal formation.

    Osteonectin - a bone glycoprotein which has the property of binding to both collagen fibres and the hydroxyapatite crystals, and thus may be important in initiating²⁶ bone mineralisation by acting²⁷ as a template for nucleation. Osteonectin is also produced by endothelial cells and plateletes and is able to bindfibrinogen.

    Osteopontin - an adhesive²⁸ glycoprotein related to sialoproteins, which is secreted by osteoclasts to assist in their adhesion to the bone surface. After bone resorption it may then act as a signal to stimulate²⁹ osteoblast activity

    Osteoporosis - a reduction in bone mass which occurs commonly in post menopausal females, but also in older men. It is due to a reduction in the activity of the ovaries and a decreased secretion³⁰ of oestrogen. Bone formation and bone healing are not affected³¹ but more bone is resorbed by osteoclasts than is replaced. Lack of exercise is also a factor in bone loss.

    Oxytalin fibres - are related to elastic³² fibres, though they have a smaller core of elastin. They are found in the periodontal ligament and in the epidermis³³ of thin skin, but not in the oral mucosa.