Occlusal wear - loss of enamel on the biting surface of the teeth due to the abrasive action of chewing natural unprocessed food.     Odontoblast process - the extension of the cy lasm of an odontoblast which remains 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 lining the dental pulp, derived from the dental papilla wich form the dentine of the tooth crown and root. New odontoblasts may become differentiated from less specialised pericytes in the pulp.     Occlusal interference - contact between opposing teeth during chewing, which prevents the other teeth touching. In extreme lateral and protrusive positions of the jaw, 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 - genes which have the capacity if expressed to cause tumour formation.     Organelle - a structures within a cell which has a specific structure or function, such as thenucleus, endoplasmic reticulum, golgi apparatus, lysosomes and mitochondria.. Cell organelles are a feature of eucaryotic cells.     Osmotic pressure - Water tends to move towards dense concentrations of ions. Sugar solutions on the surface of exposed dentine cause water to be drawn out of the dentinal tubules causing which distorts the odontoblast causing pain.     Osseous integration - a term used to describe the desired adhesion between an implant and the bone which holds it in place.     Osteoblasts - cells which differentiate from pericytes and secrete both the matrix and mineral of bone.     Osteocalcin - a calcium binding protein, synthesised by the osteoblast and secreted into the matrix at the time of bone mineralisation. Mice bread without the osteocalcin gene 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 component 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 initiating bone mineralisation by acting as a template for nucleation. Osteonectin is also produced by endothelial cells and plateletes and is able to bindfibrinogen.     Osteopontin - an adhesive 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 stimulate 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 secretion of oestrogen. Bone formation and bone healing are not affected 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 elastic fibres, though they have a smaller core of elastin. They are found in the periodontal ligament and in the epidermis of thin skin, but not in the oral mucosa.