old key
old man
old woman
om
omelette
omg
on! arrow
oncoming automobile
oncoming bus
oncoming fist
oncoming police car
oncoming taxi
one o’clock
one-storey
one-storeyed
one-thirty
on-line service
on the dole
open-air theatre
open book
open file folder
open goods truck
open hands
open line
open lock
open mailbox with lowered flag
open mailbox with raised flag
open point
open wagon
operating theatre
optical disc
optical fibre
orange book
ore-carrying wagon
organisation
organisation of work
organosulphur compound
orientate
orthodox cross
orthopaedics
orthopaedist
ostracise
outbox tray
outer home signal
out of order
over
overfertilisation
overhead railway
overlap
overseas
British English