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Oxygen deprivation (Hypoxia) – Insufficient oxygen supply to tissues, which can lead to dizziness, confusion, and difficulty breathing.
Obesity – Excessive body weight, often resulting from poor diet, lack of exercise, or metabolic issues.
Oedema (Edema) – Swelling caused by excess fluid accumulation in tissues, commonly in the legs, feet, or hands.
Osteoporosis – Weakening of bones, making them brittle and more likely to fracture, often linked to aging or nutritional deficiencies.
Out of breath (Dyspnea) – Difficulty breathing, often caused by respiratory conditions like asthma, pneumonia, or heart failure.
Oral thrush – A fungal infection in the mouth, leading to white patches and discomfort.
Ocular discomfort – Eye strain, dryness, or irritation, often from prolonged screen time or eye conditions.
Overheating (Hyperthermia) – Elevated body temperature, often due to exercise, hot weather, or illness.
Onychomycosis – Fungal infection of the nails, causing them to become discolored, thickened, or brittle.
Otitis (Ear infection) – Inflammation or infection of the ear, causing pain, hearing loss, and sometimes fever.

Obsessive thoughts – Persistent, intrusive thoughts that can lead to anxiety or compulsive behaviors, commonly seen in obsessive-compulsive disorder (OCD).
Oppositional behavior – Defiant or resistant attitude, often seen in children with oppositional defiant disorder (ODD).
Overthinking – Continuously analyzing or dwelling on something, often leading to anxiety or indecision.
Overwhelm – A feeling of being mentally or emotionally overloaded, often resulting in stress, anxiety, or exhaustion.
Obsessive-compulsive behavior – Repetitive, compulsive actions or rituals that someone feels driven to perform to reduce anxiety.
Outbursts of anger – Sudden, intense emotional reactions, often accompanied by aggression or frustration.
Overconfidence – Excessive belief in one's abilities, sometimes leading to reckless behavior or decision-making.
Ornery behavior – Irritable, stubborn, or difficult behavior, often due to frustration or discomfort.
Odd beliefs – Holding unconventional or irrational ideas, which may be part of a larger mental health condition like delusional disorder or schizophrenia.