This article contains a list of commonly used medical abbreviations and acronyms that start with the letter O and may be encountered in medicine and radiology (please keep the main list and any sublists in alphabetic order). 

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

• O

  • blood type (ABO system)

  • oxygen

• O&G: obstetrics and gynecology

• OA: osteoarthritis

• OAB: overactive bladder

• OAC: oral anticoagulant

• OAML: orbital adnexal MALT lymphoma

• OAR: organ at risk

• OAS: orbital apex syndrome

• OAT

  • older adult trauma

  • osteochondral autograft transplantation 

• OATS: oligoasthenoteratozoospermia

• OB/GYN: obstetrics and gynecology

• Obl: oblique

• OBO: on background of

• Obs: observations

• OC

  • optic canal

  • optic chiasm

• OCA: osteochondral allograft transplantation

• OCD

  • obsessive-compulsive disorder

  • osteochondral defect

  • osteochondritis dissecans

• OCF

  • occipital condyle fracture

  • odontogenic cutaneous fistula

• OCG: oral cholecystogram

• OCI: osteitis condensans ilii

• OCL: osteochondral lesion

• OCP

  • octacalcium phosphate

  • oral contraceptive pill

  • ova, cysts and parasites

• OCS: otic capsule sparing (temporal bone fractures)

• OCT: optical coherence tomography

• OCV: otic capsule violating (temporal bone fractures)

• OD

  • occupational disease

  • once daily (Latin: omne in die)

  • osteochondritis dissecans

  • overdose

  • right eye (Latin: oculus dexter)

• ODT: oroduodenal tube

• OE: otitis externa

• OE-MRI: oxygen-enhanced magnetic resonance imaging

• O/E: on examination

• OFC: osteitis fibrosa cystica

• OFD: occipitofrontal diameter

• OFDI: optical frequency domain imaging

• Og: oganesson

• EGD: esophagogastroduodenoscopy

• OGS: orthognathic surgery

• OGT: orogastric tube

• OHCA: out-of-hospital cardiac arrest

• OHS

  • obesity hypoventilation syndrome

  • ovarian hyperstimulation syndrome

• OHSS: ovarian hyperstimulation syndrome

• OHT: orthotopic heart transplant

• OHTx: orthotopic heart transplant

• OI

  • opportunistic infection

  • osteogenesis imperfecta

• OLB: open lung biopsy

• OLIF: oblique lumbar interbody fusion

• OLLIF: oblique lateral lumbar interbody fusion

• OLTx: orthotopic liver transplant

• OM

  • obtuse marginal artery

  • occipitomental

  • osteomalacia

  • osteomyelitis

  • otitis media

  • overlap myositis

• om: every morning (Latin: omni mane)

• OMC: ostiomeatal complex

• OMU: ostiomeatal unit

• OMX: osteochondromyxoma

• ON

  • take every night (drug prescriptions, Latin: omni nocte)

  • optic nerve

  • optic neuritis

• ONH: optic nerve head

• O/N: overnight

• ONG: optic nerve glioma

• ONSD: optic nerve sheath diameter

• OOA: esophageal obturator airway

• OOB: out of bed

• OOC: orthokeratinized odontogenic cyst

• OOH

  • out-of-hospital

  • out-of-hours

• OOHCA: out-of-hospital cardiac arrest

• OOHVFA: out-of-hospital ventricular fibrillation arrest

• OOP: out-of-plaster

• OP

  • operation

  • opinion

  • opposed phase

  • osteoporosis

  • out of phase

  • outpatient

• OPARACHEE: on resonance paramagnetic chemical exchange effect

• OPD: outpatient department

• OPG: orthopantomogram

• OPK: osteopoikilosis

• OPL: oblique posterior ligament

• OPLL: ossification of the posterior longitudinal ligament

• OPPT: oriented to person, place, and time

• OPT: orthopantomogram

• OPV: oral polio vaccine

• OR

  • odds ratio

  • operating room

  • organ at risk

• Ordercomms: order communications

• ORF: open reading frame (in protein synthesis)

• ORIF: open reduction internal fixation

• ORN: osteoradionecrosis

• ORSA: oxacillin-resistant staphylococcus aureus

• ORT: oral rehydration therapy

• OS: left eye (Latin: oculus sinister)

• Os: osmium

• OSA: obstructive sleep apnea

• OSD

  • occult spinal dysraphism

  • Osgood-Schlatter disease

• Osm

  • osmole

  • osmolarity

• OSNA: one step nucleic acid amplification

• OSP: oncocytic sinonasal papilloma

• OT

  • occupational therapy

  • optic tract

  • oxytocin

• OTC

  • oesophagotracheal combitube

  • on-table cholangiogram

  • over-the-counter (medication)

• OTD

  • organ and tissue donation

  • out the door (discharge)

• OTTR: organ transplant tracking record

• OU: both eyes (Latin: oculus uterque)

• OV

  • esophageal varices

  • office visit

• oVEMP: ocular vestibular-evoked myogenic potentials

• OWR: Osler-Weber-Rendu syndrome

• OxyHb: oxygenated hemoglobin

• OYL: ossification of the yellow ligament

• oz: ounce (Italian: onza)