Tree terms: o
- oak apple
- oak galls
- oak jewel beetle
- oak leaf phylloxera
- oak mildew
- oak pinhole borer (Platypus cylindrus)
- oak processionary moth
- oak wilt
- obiter
- occlusion
- occupancy
- Occupiers Liability Act 1957 (as amended 1984)
- ocean conveyor (global conveyor)
- oedometer
- offences
- Office of Qualifications and Examinations Regulation
- officer
- Ofqual
- oil
- oil separator
- old growth woodland
- Olympic park gardens
- ombudsman
- one-year shoot
- open churchyard
- open country
- open pollination
- open space
- Open Spaces Society
- open-board fence
- open-centre
- open-grown
- orchard
- ordinary watercourse
- ore pit
- organic
- organic matter
- organic soils
- organic standard
- oriental chestnut gall wasp
- origin
- ornamental
- osier
- osmosis
- osmotic pressure
- otter
- outline planning application
- outplanting
- oven-dry
- overage
- overconsolidated clay
- overhanging branches
- over-mature
- overstorey
- overtop
- ovule
- own-root tree
- oxygen
- ozone
- ozone hole
- ozone layer