Medical Abbreviations Starting With "O"

This page lists 138 medical abbreviations starting with "O", each with its full form and a plain-English meaning. Where one abbreviation has more than one common meaning, each meaning has its own row — context in the chart is what tells them apart. Terms flagged ⚠ Error-prone are on recognised error-prone lists and should be written out in full. Use the filter box to narrow this list, the search on the main guide to search all 5,715 entries, or jump to another letter:

A B C D E F G H I J K L M N 0–9 P Q R S T U V W X Y Z

Abbreviation	Full form	What it means	Used in
O-arm	O-arm imaging system	Intraoperative 3D imaging for spine surgery	Radiology & Imaging Surgery Musculoskeletal
O-RADS	Ovarian-Adnexal Reporting and Data System	Standard categories for ovarian imaging	Radiology & Imaging Obstetrics & Gynecology
O2	oxygen	Gas essential for the body's energy production	Respiratory
O2 concentrator	oxygen concentrator (device)	Machine making oxygen from room air	Respiratory Nursing
OA	occiput anterior	Baby's head position with the back of the head toward the mother's front	Obstetrics & Gynecology
OA	osteoarthritis	Joint disease where cartilage wears down over time	Musculoskeletal
OAC	oral anticoagulant	Blood-thinning medicine taken by mouth	Cardiology
OAE	otoacoustic emissions (test)	Ear sound response test — test	Neurology Pediatrics
OAE/ABR pathway	OAE ABR screening pathway (two-step)	Two-technology newborn hearing pathway	Pediatrics
OAGB	one anastomosis gastric bypass	Single-join gastric bypass	Surgery Gastroenterology
OAR	organ at risk	Healthy organ near a radiation target that must be spared	Radiology & Imaging Hematology & Oncology
OASIS	obstetric anal sphincter injury	Third or fourth degree tear group	Obstetrics & Gynecology Surgery
OATS	osteochondral autograft transfer (OATS)	Moving a cartilage plug to a defect	Surgery Musculoskeletal
OB	obstetrics	Care of pregnancy and childbirth	Obstetrics & Gynecology
obesity child	obesity (BMI ≥95th percentile)	Child weight category	Pediatrics Endocrinology
OBIN	obinutuzumab (chart shorthand)	CD20 antibody shorthand	Prescription & Medication Orders Hematology & Oncology
OBS	observation	Watching a patient over time for changes	General Medical Nursing
Obs chart	observation chart	Chart recording vital signs over time	Nursing Vitals & Assessment
obturator	obturator (trach introducer)	Guide used when inserting a trach tube	Nursing Respiratory Surgery
obturator n	obturator nerve (anatomy)	Inner thigh nerve	Neurology Musculoskeletal
OCB	oligoclonal bands	Antibody bands in spinal fluid suggesting immune activity	Neurology Laboratory
occipital lobe	occipital lobe (anatomy)	Back brain region for vision	Neurology
occipital neuralgia	occipital neuralgia (diagnosis)	Back-of-head nerve pain — diagnosis	Neurology
OCD	obsessive-compulsive disorder	Condition with intrusive thoughts and repetitive behaviours	Psychiatry
OCP	oral contraceptive pill	Birth control medicine taken by mouth	Prescription & Medication Orders Obstetrics & Gynecology
OCPD	obsessive-compulsive personality disorder	Personality pattern of perfectionism and need for control	Psychiatry
OCT	optical coherence tomography	Light-based scan imaging the layers of the retina	Radiology & Imaging General Medical
OCTA	optical coherence tomography angiography	Retinal blood flow imaging without dye	General Medical Radiology & Imaging
octreotide scan	somatostatin receptor scintigraphy	Nuclear scan for neuroendocrine tumors	Radiology & Imaging Endocrinology Hematology & Oncology
OD	overdose (presentation)	Drug excess presentation — presentation	Emergency & Critical Care Psychiatry Prescription & Medication Orders
OD ⚠ Error-prone	right eye	The right eye, from Latin oculus dexter	General Medical
ODD	oppositional defiant disorder	Childhood pattern of angry, defiant behaviour toward authority	Psychiatry Pediatrics
ODI	Oswestry Disability Index	Back pain disability questionnaire	Musculoskeletal
ODI	oxygen desaturation index	Number of oxygen dips per hour during sleep	Respiratory
odontoid child	synchondrosis (odontoid growth plate)	Growth plate of the neck peg bone in children	Pediatrics Radiology & Imaging Musculoskeletal
ODS	osmotic demyelination syndrome	Brain injury from correcting sodium too fast	Neurology Renal & Nephrology Emergency & Critical Care
ODT	orally disintegrating tablet	Tablet that dissolves on the tongue	Prescription & Medication Orders
OE	otitis externa	Infection or inflammation of the outer ear canal	General Medical Infectious Disease
OESIL	Osservatorio Epidemiologico sulla Sincope nel Lazio score	Syncope risk score	Cardiology Emergency & Critical Care
OFC	occipitofrontal circumference (abbreviation)	Head circumference abbreviation	Pediatrics Neurology
OG	orogastric	Through the mouth into the stomach, as a tube	Nursing Gastroenterology
OG tube	orogastric tube (device)	Mouth-to-stomach tube	Nursing Gastroenterology Emergency & Critical Care
OGD	oesophago-gastro-duodenoscopy	Camera examination of food pipe, stomach and upper intestine	Gastroenterology Surgery
OGTT	oral glucose tolerance test	Sugar drink test for diabetes	Endocrinology Laboratory Obstetrics & Gynecology
OH	orthostatic hypotension (measurement)	Standing blood pressure drop — measurement	Neurology Cardiology Nursing
OHCA	out-of-hospital cardiac arrest	Cardiac arrest occurring outside a hospital	Cardiology Emergency & Critical Care
OHS	obesity hypoventilation syndrome	Under-breathing linked to obesity causing high carbon dioxide	Respiratory
OHSS	ovarian hyperstimulation syndrome	Over-response of ovaries to fertility drugs	Obstetrics & Gynecology Emergency & Critical Care
OI	opportunistic infection	Infection taking advantage of a weakened immune system	Infectious Disease
OI	osteogenesis imperfecta	Inherited condition causing bones that break easily	Musculoskeletal Pediatrics
Oint	ointment	Greasy preparation applied to the skin	Prescription & Medication Orders
OIT	oral immunotherapy	Gradual food dosing to reduce allergic reactions	General Medical
OLAP	olaparib (chart shorthand)	PARP blocker pill shorthand	Prescription & Medication Orders Hematology & Oncology
OLIF	oblique lumbar interbody fusion	Angled front-side back fusion	Surgery Musculoskeletal
oligomenorrhea	oligomenorrhea (infrequent periods)	Periods more than 35 days apart — term	Obstetrics & Gynecology Endocrinology
olive mass	olive mass (pyloric palpation)	Felt lump of the thickened outlet	Pediatrics Gastroenterology
Olsen chart	Olsen growth curves (preterm)	Alternative preterm growth reference	Pediatrics
OLT	orthotopic liver transplant	Liver transplant placed in the normal position	Surgery Gastroenterology
OM	obtuse marginal artery	Branch artery supplying the side of the heart	Cardiology
OM	osteomyelitis	Infection inside a bone	Musculoskeletal Infectious Disease
OM	otitis media	Infection or inflammation of the middle ear	General Medical Infectious Disease Pediatrics
OME	otitis media with effusion	Fluid in the middle ear without acute infection	General Medical Pediatrics
omental flap	omental flap	Abdominal fat apron moved to cover a defect	Surgery
omentectomy	omentectomy (cancer staging)	Removing the fatty apron in ovarian cancer surgery	Obstetrics & Gynecology Surgery Hematology & Oncology
omentum anatomy	omentum (anatomy)	Fatty apron over the bowel — anatomy	Gastroenterology Surgery
OMFS	oral and maxillofacial surgery	Surgical specialty of the mouth, jaws and face	Surgery
omphalitis	omphalitis (cord infection)	Infected umbilical stump	Pediatrics Infectious Disease Emergency & Critical Care
omphalitis child	omphalitis (infection)	Cord stump infection	Pediatrics Infectious Disease
omphalocele	omphalocele (defect)	Organs in a sac at the cord — defect	Pediatrics Surgery Obstetrics & Gynecology
ON	optic neuritis	Inflammation of the optic nerve causing vision loss	Neurology
ON	osteonecrosis	Bone tissue death from loss of blood supply	Musculoskeletal
on-call	on-call (availability)	Available to be called in for work	Nursing General Medical Surgery
oncoplastic	oncoplastic breast surgery	Cancer removal combined with plastic reshaping	Surgery Hematology & Oncology
one-and-a-half	one-and-a-half syndrome (eye)	Brainstem lesion causing combined gaze palsy	Neurology
one-time order	single dose order	Order given once only	Nursing Prescription & Medication Orders
ONH	optic nerve head	Front of the optic nerve seen in the eye	General Medical Neurology
onset-to-door	onset-to-door time (stroke metric)	Symptom start to arrival time	Neurology Emergency & Critical Care
OOB	out of bed	Patient is up out of bed	Nursing
OOB ad lib	out of bed as desired	Free mobilization order	Nursing
OP	occiput posterior	Baby's head position with the back of the head toward the mother's back	Obstetrics & Gynecology
OPA	oropharyngeal airway	Curved device keeping the tongue off the airway	Emergency & Critical Care
Open appy	open appendicectomy	Appendix removal through a larger cut	Surgery Gastroenterology
open chole	open cholecystectomy	Gallbladder removal through an open incision	Surgery Gastroenterology
opening pressure	opening pressure (LP measurement)	Spinal fluid pressure at tap — measurement	Neurology Laboratory
oPEP	occupational post-exposure prophylaxis	HIV prevention after a workplace exposure	Infectious Disease Prescription & Medication Orders
ophthalmic art	ophthalmic artery	First internal carotid branch, feeding the eye	Neurology
opioid exposure child	opioid exposure (household)	Child contact with household opioids emergency	Pediatrics Emergency & Critical Care
Oppenheim	Oppenheim sign (reflex)	Shin stroke eliciting an upgoing toe	Neurology
OPQRST	onset, provocation, quality, radiation, severity, time	Pain history mnemonic used in assessment	Emergency & Critical Care Vitals & Assessment
opsoclonus	opsoclonus-myoclonus syndrome (dancing eyes)	Chaotic eye movements linked to neuroblastoma	Pediatrics Neurology Hematology & Oncology
OPU	oocyte pick-up (egg retrieval)	Collecting eggs for IVF	Obstetrics & Gynecology
OPV	oral poliovirus vaccine	Polio vaccine given by mouth	Pediatrics Infectious Disease
OR	odds ratio	Measure comparing odds between two groups	General Medical Laboratory
OR	operating room	Room where operations are performed	Surgery
oral aversion	oral aversion (sensory)	Avoidance of mouth experiences after tubes or procedures	Pediatrics Nursing
oral food challenge	oral food challenge (procedure)	Supervised feeding test for allergy — procedure	Pediatrics Infectious Disease
ORBIT	ORBIT bleeding score	Bleeding risk score in atrial fibrillation	Cardiology
orbital	orbital cellulitis (emergency)	Eye socket infection emergency	Pediatrics Infectious Disease Emergency & Critical Care Surgery
orchiectomy	orchiectomy (procedure)	Removal of a testicle	Surgery Renal & Nephrology
ORIF	open reduction and internal fixation	Operation aligning a broken bone and fixing it with hardware	Musculoskeletal Surgery
ORIF ankle	open reduction internal fixation (ankle)	Plate and screw repair of an ankle fracture	Surgery Musculoskeletal
ORIF wrist	open reduction internal fixation (wrist)	Plate repair of a wrist fracture	Surgery Musculoskeletal
ORR	objective response rate	Proportion of patients whose tumours shrink on treatment	Hematology & Oncology
ortho dental	orthodontics (shorthand)	Teeth-straightening specialty shorthand	General Medical
orthorexia	orthorexia (proposed term)	Unhealthy obsession with healthy eating	Psychiatry
OS ⚠ Error-prone	left eye	The left eye, from Latin oculus sinister	General Medical
OS	overall survival	Length of time patients live after diagnosis or treatment	Hematology & Oncology
OSA	obstructive sleep apnea	Repeated pauses in breathing during sleep from a blocked airway	Respiratory
OSA child	pediatric obstructive sleep apnea (diagnosis)	Sleep airway blockage in children — diagnosis	Pediatrics Respiratory Surgery
OSAHS	obstructive sleep apnea-hypopnea syndrome	Full name for obstructive sleep apnea syndrome	Respiratory General Medical
OSE	oseltamivir (chart shorthand)	Influenza antiviral chart shorthand	Prescription & Medication Orders Infectious Disease
Osgood	Osgood-Schlatter disease (knee)	Teen kneecap tendon bump pain	Pediatrics Musculoskeletal
OSI	osimertinib (chart shorthand)	EGFR blocker pill shorthand	Prescription & Medication Orders Hematology & Oncology
Osm	osmolality	Concentration of dissolved particles in blood or urine	Laboratory Renal & Nephrology
osm gap	osmolal gap	Difference between measured and calculated osmolality	Laboratory Emergency & Critical Care Renal & Nephrology
ossiculoplasty	ossiculoplasty (hearing bone repair)	Reconstruction of the middle ear bones	Surgery
osteocalcin	osteocalcin (bone formation marker)	Protein reflecting bone formation	Laboratory Endocrinology Musculoskeletal
osteosarcoma	osteosarcoma (adolescent)	Teen bone cancer	Pediatrics Musculoskeletal Hematology & Oncology
ostomy appliance	ostomy appliance (pouch)	Bag worn over a stoma	Nursing Gastroenterology Surgery
OT	occupational therapist	Clinician helping people manage daily activities	General Medical
OT	occupational therapy	Therapy helping people manage daily activities	General Medical
OT	operating theatre	Room where operations are performed	Surgery
OTC	over the counter	Medicine available without a prescription	Prescription & Medication Orders
OTC def	ornithine transcarbamylase deficiency	Commonest urea cycle disorder	Pediatrics Laboratory Emergency & Critical Care
OTCD	ornithine transcarbamylase deficiency	Inherited disorder of ammonia removal	Pediatrics Laboratory
otorrhea tube	tube otorrhea (drainage)	Ear drainage through a tube	Pediatrics Infectious Disease
OU ⚠ Error-prone	both eyes	Both eyes, from Latin oculus uterque	General Medical
OUD	opioid use disorder	Problematic pattern of opioid use	Psychiatry
OV	oesophageal varices	Swollen veins in the food pipe from liver pressure	Gastroenterology
ovarian drilling	ovarian drilling (PCOS surgery)	Small burns on the ovary restoring ovulation	Obstetrics & Gynecology Surgery Endocrinology
OVD	operative vaginal delivery	Vaginal birth assisted with forceps or a vacuum device	Obstetrics & Gynecology
oVEMP	ocular vestibular evoked myogenic potential	Balance organ test using eye muscle response	General Medical Neurology
overheating	overheating (sleep risk)	Over-bundling as a sleep risk	Pediatrics Nursing
oversupply	oversupply (hyperlactation)	Making more milk than the baby needs	Obstetrics & Gynecology Nursing
overweight child	overweight (BMI 85-95th percentile)	Child weight category	Pediatrics Endocrinology
OXA-48	OXA-48 carbapenemase	Carbapenem-destroying enzyme type	Infectious Disease Laboratory
OXALI	oxaliplatin (chart shorthand)	Platinum chemotherapy shorthand	Prescription & Medication Orders Hematology & Oncology
oz	ounce	Unit of weight or fluid volume	Prescription & Medication Orders General Medical

Educational reference only. This guide is for learning and quick reference. Abbreviations vary between hospitals, countries and specialties, and some are discouraged because they can be misread. Always write and interpret orders using your own institution’s approved abbreviation list, and check anything unclear with the prescriber or your local policy.

Medical Abbreviations Starting With "O"

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