Obesity – A condition of excessive body fat that increases the risk of health issues and limits performance.

Objective data – Information that is measurable and observable, such as heart rate or sprint time.

Observation – The act of closely monitoring performance for feedback or analysis.

Observer bias – A distortion in data recording due to the observer’s personal expectations or beliefs.

Obstructive lung disease – A condition where airflow is limited, making breathing more difficult during exercise.

Occipital lobe – The part of the brain responsible for visual processing.

Occlusion training – Low-load strength training combined with restricted blood flow to enhance muscle growth.

Occupational physical activity – Activity that occurs as part of one's job, contributing to overall energy expenditure.

Octane rating – A measure of a fuel’s ability to resist knock; metaphorically used to describe high-performance athletes.

Off-season – The phase between competitive seasons used for recovery and general conditioning.

Offensive strategy – Planned actions designed to score or gain advantage in invasion games.

Offsetting fatigue – Strategies such as active recovery or nutrition to delay performance decline.

Olfaction – The sense of smell, sometimes involved in psychological responses in sport environments.

Oligodendrocyte – A type of nervous system cell that forms the myelin sheath in the brain.

Oligosaccharide – A short-chain carbohydrate used as a fuel source during moderate-intensity exercise.

Olympic lifts – Weightlifting movements like the clean and jerk or snatch, used to develop power.

Olympic periodisation – Structuring long-term training across the 4-year Olympic cycle.

One-rep max (1RM) – The maximum weight a person can lift once with correct technique.

One-touch passing – A football technique involving immediate passing without stopping the ball.

Onset of blood lactate accumulation (OBLA) – The point where lactate builds up rapidly in the blood.

Onset of fatigue – The moment during performance when physical or mental tiredness begins to affect output.

Open chain exercise – Movements where the distal limb moves freely, such as leg extensions.

Open kinetic chain – A biomechanical concept where the end segment of a limb is not fixed.

Open loop control – A motor control theory where movement is pre-planned and not adjusted by feedback.

Open motor skill – A skill performed in a dynamic, unpredictable environment, like dribbling in football.

Open skill – A skill influenced by changing environments and opponents.

Open stance – A body position where the feet are placed apart for better balance and agility.

Open-water swimming – Competitive or recreational swimming in natural bodies of water.

Operating range – The optimal physiological zone in which the body or system performs efficiently.

Operation of lever – How bones and muscles act together to produce movement through mechanical advantage.

Opponent process theory – A psychological model explaining emotional swings before and after competition.

Opportunity cost – The benefits lost when choosing one training method over another.

Optic nerve – The nerve transmitting visual information from the eye to the brain.

Optimal arousal – The level of mental alertness that leads to best performance.

Optimal load – The resistance that produces maximum power or desired adaptation in training.

Optimal movement pattern – A biomechanically efficient and safe way of performing a skill.

Optimisation – Refining technique, strategy, or physiology to achieve peak performance.

Oral glucose tolerance test – A test of how the body processes sugar, sometimes used in athlete health screening.

Oral rehydration solution – A fluid replacement formula used to treat dehydration during prolonged exercise.

Orbitofrontal cortex – A part of the brain involved in decision-making, risk-taking, and reward during sport.

Order of play – The sequence of events or matches in a tournament or competition.

Organ – A structure composed of tissues that performs a specific function, such as the heart or lungs.

Organisation – The way training, practice or teams are structured to maximise performance.

Organised sport – Structured, rule-based sport usually involving formal competition.

Organelle – A structure within a cell that performs specific functions, like the mitochondria.

Organism – An individual living system; in sport science, refers to the whole athlete.

Orogenesis – While typically a geological term, metaphorically used to describe the ‘building’ of an athlete through stress and adaptation.

Orthopaedic – Relating to the musculoskeletal system and its treatment or rehabilitation.

Orthotic – A device used to support or align the foot and lower limbs during activity.

Oscillation – A repeated back-and-forth movement, like in dynamic stretching or certain biomechanical actions.

Oscillatory training – Resistance training using vibrating or bouncing movements to challenge stabilisers.

Outcome goal – A goal focused on the end result, like winning a race.

Outcome measure – A variable used to evaluate the success of a training programme or intervention.

Outer range – The limit of a joint’s movement, often used in flexibility assessment.

Outfield player – Any team sport athlete who is not the goalkeeper, such as defenders, midfielders, or forwards.

Outlier – A data point significantly different from others, possibly indicating error or unique performance.

Out-of-bounds – In sport, when the ball or athlete goes outside the designated playing area.

Overarm throw – A fundamental movement pattern used in sports like cricket or baseball.

Overcompensation – The body’s response to stress that results in improved performance after recovery.

Overhead squat – A strength and mobility exercise used to assess flexibility and stability.

Overhydration – Excess fluid intake that can disturb electrolyte balance and reduce performance.

Overload – A training principle where stimulus must exceed normal levels to cause adaptation.

Overpronation – Excess inward rolling of the foot during running, potentially causing injury.

Overreaching – Short-term training stress that leads to temporary fatigue but improved performance after rest.

Overstriding – A running fault where the foot lands too far ahead, reducing efficiency.

Overtraining – Excessive training without adequate recovery, leading to performance decline and fatigue.

Overuse injury – Damage caused by repeated stress on a body part without sufficient recovery.

Oxidation – A chemical reaction involving oxygen that helps release energy from nutrients.

Oxidative capacity – The ability of muscles to use oxygen efficiently during aerobic exercise.

Oxidative phosphorylation – The main process of ATP production in the mitochondria during aerobic metabolism.

Oxidative stress – An imbalance between free radicals and antioxidants, often linked to fatigue or overtraining.

Oxygen – A gas essential for aerobic energy production in muscles.

Oxygen consumption (VO₂) – The amount of oxygen used by the body during exercise.

Oxygen deficit – The shortfall between oxygen supply and demand at the start of exercise.

Oxygen debt – The extra oxygen consumed during recovery to restore the body to resting state.

Oxygen saturation – The percentage of oxygen bound to haemoglobin in the blood.

Oxyhaemoglobin – Haemoglobin bound to oxygen, used to transport oxygen in the bloodstream.

Oxytocin – A hormone linked to bonding and social trust, increasingly explored in team dynamics.

Oblique muscles – Abdominal muscles responsible for rotation and side bending of the torso.

Occiput – The back part of the skull, relevant in postural and impact injury studies.

Open circuit spirometry – A method for measuring oxygen consumption during exercise.

Operating point – The balance point in physiological systems, such as heart rate at submaximal load.

Opportunity window – A time frame when specific training adaptations are most responsive.

Orbital plane – A curved motion path relevant in certain throwing or rotational sports.

Orgasmic threshold – A concept explored in exercise-induced euphoria (runner’s high); niche in sport psychology.

Orientation – A spatial awareness skill important in gymnastics, parkour, and swimming.

Organisational culture – The shared beliefs and values within a sports team or organisation.

Outcome variable – A dependent variable measured to assess performance or training impact.

Outfield conditioning – Fitness training for non-goalkeeper players in team sports.

Oxygen cascade – The stepwise reduction of oxygen partial pressure from air to muscle cells.

Oxaloacetate – A molecule in the Krebs cycle critical for aerobic metabolism.

Oblique strain – A common injury in rotational sports affecting the side abdominal muscles.

Open water buoyancy – A factor influencing swim technique and speed in triathlon or open water swimming.

Osmolality – A measure of solute concentration in body fluids, affecting hydration and absorption.

Osmoregulation – The body’s process of maintaining fluid and electrolyte balance.

Osteoblast – A bone-forming cell important in growth, repair, and adaptation to load.

Osteoclast – A cell that breaks down bone tissue during remodelling.

Osteocyte – A mature bone cell that helps regulate bone maintenance and mineral content.

Osteogenesis – The process of bone formation, stimulated by weight-bearing exercise.

Osteopenia – A condition of lower-than-normal bone density, increasing fracture risk.

Osteoporosis – A disease where bones become brittle and weak due to low bone density.

Osteoarthritis – A degenerative joint condition affecting athletes due to wear and tear.

Overhead press – A resistance exercise targeting shoulder and upper back strength.