David Appleyard's English Language User Guides and References
Index of Americanisms
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AmericanAmerican BritishBritish
oatmeal porridge
ocean (American usage of 'the ocean' is broader than in
            Britain and often connotes the sea in general: 
            'From my hotel room I can see the ocean!')
ocean (meaning one of the five major
             masses of water in the world);
sea (as in: 'From my hotel room I can see the sea!')
ocean liner liner, cruise liner
odometer mileometer, milometer
off-ramp (to exit a highway or freeway) slip road
off-the-rack (clothes ready-made, ready-to-wear) off-the-peg
off the wall [colloq.] unconventional
oftentimes often
on-ramp (to access a highway or freeway) slip road
on sale for sale at reduced price, in a (bargain) sale
on the weekend, on New Year's, on Christmas, etc. at the weekend, at New Year, at Christmas, etc.
on vacation on holiday
one-family house detached house
one-way ticket single (ticket)
open-pit mining, open-cut mining opencast mining
operating room (in hospital) operating theatre
optometrist optician
orchestra seats (in theater) stalls
orient [v.] orientate [v.]
out of whack [colloq.]   not looking right or working as it should
out to lunch [colloq.] (crazy) off one's rocker [slang]
outbox (on desk) out-tray
outhouse outdoor lavatory, carsy [slang]
outlet (electrical) wall socket, mains socket, plug socket, plug [colloq.], power point
oven mitt oven glove
overalls (jeans or tough denim trousers with cloth
              covering chest and straps over shoulders)
dungarees
overhead [U] or [C] overheads (plural noun)
overpass (bridge carrying one road over another) flyover

 

Index of Americanisms
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   |   British-American