- Old Forge AirportIdent: NK26Serves: Old Forge
- Old Forker UltralightportIdent: TA30Serves: Sante Fe
- Old Fort Farm AirportIdent: 50NYServes: Hemlock
- Old Glory FieldIdent: US-8437Serves: Fort Wayne
- Old Hickory AirparkIdent: US-6204Serves: Geneva
- Old Hickory Ranch AirportIdent: 56LAServes: Keithville
- Old Hoppe Place AirportIdent: 67TXServes: Agua Dulce
- Old Kingsbury AerodromeIdent: 85TEServes: Kingsbury
- Old Orchard AirparkIdent: 2NK9Serves: Modena
- Old Orchard AirportIdent: 0PS4Serves: Curwensville
- Old Plains AirportIdent: 9PA2Serves: Pennsburg
- Old Port Royal AirportIdent: 6NY1Serves: Corning
- Old Reb AirportIdent: 5XS6Serves: Mercedes
- Old Rhinebeck AerodromeIdent: NY94Serves: Rhinebeck
- Old Valley AirportIdent: US-0513Serves: Sharps Chapel
- Oldfield AeroIdent: 27IAServes: Keosauqua
- Oldham County AirportIdent: KE52Serves: Vega
- Oldstown Flying Tails AirportIdent: US-0903Serves: Cortland
- Oleo AirportIdent: 4II3Serves: Fillmore
- Olfert AirportIdent: MT04Serves: Lustre
- Olinger AirparkIdent: OR81Serves: Hillsboro
- Olive Branch AirportIdent: KOLVServes: Olive Branch
- Oliver Landing AirportIdent: 42TNServes: Paris
- Oliver Springs Inc AirportIdent: TN08Serves: Oliver Springs
- Olivia Regional AirportIdent: KOVLServes: Olivia
- Olmstead Landing StripIdent: 73NYServes: Masonville
- Olmstead Sky Ranch AirportIdent: ID25Serves: Sandpoint
- Olney Municipal AirportIdent: KONYServes: Olney
- Olney Noble AirportIdent: KOLYServes: Olney-Noble
- Olson AerodromeIdent: SN40Serves: Derby
- Olson FieldIdent: NE30Serves: Holdrege
- Olson's AirportIdent: 3WI1Serves: Union Grove
- Oltman-Shuck AirportIdent: 83ILServes: Minonk
- Olustee Municipal AirportIdent: F09Serves: Olustee
- Olympic FieldIdent: WA45Serves: Discovery Bay/Maynard
- Olympus AirportIdent: 38TTServes: Campbell
- Omak AirportIdent: KOMKServes: Omak
- Omar N Bradley AirportIdent: KMBYServes: Moberly
- Omega 1 AirportIdent: 12COServes: Hotchkiss
- Omni AirparkIdent: 6NY2Serves: Horseheads
- Omni AirportIdent: LA46Serves: Port Allen
- Omni Ranch Ultralight FlightparkIdent: 49XAServes: Edinburg
- OMS UAS Test Range AirportIdent: US-9264Serves: Kingman
- On the Rocks AirportIdent: 1CA6Serves: Alpine
- Ona AirparkIdent: K12VServes: Milton
- One Eyed Creek AirportIdent: US-4465Serves: Lufkin
- One Grand FieldIdent: 5TN9Serves: Cookeville
- One Oak AirportIdent: 77KYServes: Mount Sterling/Owingsville
- One Particular Harbour Ultralight FlightparkIdent: US-4849Serves: Deming
- One Shot AirportIdent: OH09Serves: Howard
- One Sixty Four Place AirportIdent: TS02Serves: Jacksboro
- Oneida Lake Strip AirportIdent: US-0759Serves: Oneida
- Onida Municipal AirportIdent: K98DServes: Onida
- Onion Crest AirparkIdent: 43NEServes: Kearney
- Onley AirportIdent: VG20Serves: Onley
- Ontonagon County Schuster FieldIdent: KOGMServes: Ontonagon
- Onyon AirportIdent: VT43Serves: Townshend
- Oolenoy Valley AirportIdent: SC75Serves: Pickens
- Oostburg AirportIdent: US-9079Serves: Oostburg
- Open A-1 Ranch AirportIdent: 3LA8Serves: Lake Charles