O
- Oakington: TL414645
- Odd Quarter: TL185804
- Odsey: TL292382
- Offord Cluny: TL220672
- Offord D'Arcy: TL219663
- Old Hurst: TL300770
- Old Weston: TL100775
- Oldfield Pond: TF132002
- Open Coppice: TF121035
- Orchard Park: TL4461
- Orton BP: TL170935
- Orton Brimbles: TL150960
- Orton Goldhay: TL160953
- Orton Lock: TL167972
- Orton Longueville: TL165965
- Orton Malborne: TL167960
- Orton Meadows: TL162968
- Orton Mere: TL167970
- Orton Southgate: TL143948
- Orton Water: TL165968
- Orton Waterville: TL160960
- Orton Wistow: TL145965
- Orwell: TL362503
- Orwell Clunch Pit: TL363506
- Ouse Fen: TL375725
- Ouse Washes (Earith): TL395755
- Ouse Washes (Jolly Bankers): TL409777
- Ouse Washes (Mepal): TL436815
- Ouse Washes (Pymoor): TL501885
- Ouse Washes (railway to Norfolk): TL513919
- Ouse Washes (Sutton Gault): TL428794
- Ouse Washes (The Gullet) - See Sutton Gault (Gullet Marsh)
- Ouse Washes (The Gravel): TL456824
- Ouse Washes Reserve: TL471860
- Out Wood: TL660550
- Over Fen/GP - See Ouse Fen
- Over Railway Cutting NR: TL382687
- Overcote: TL355715
- Overhall Grove: TL337633
- Overton Lake - See Ferry Meadows CP
- Oxey Wood: TF123036
- Oxlode: TL486862