-N-

Nappy:
   Diaper

National:
    Federal
(as in Federal laws, the Federal Bureau of Investigation, etc.)

National Insurance Number:
    Social Security Number

Newsagent:
    Newsstand

The 'Nick', Prison:
   Prison, State Penitentiary, the 'Slammer' and various other slang names...

Nil:
    Nothing, Zero

Nought:
    Zero

Noughts & Crosses:
    Tic-Tac-Toe

Note (money):
    Bill 
("I found a $20 bill on the street last night." "Wow.")

Number Plates:
    License Plates
(As it should be.. Why do you Brits call it a 'number plate' when letters are also included??)

OAP:
    Senior Citizen

Odd Socks:
    Mis-Matched socks
("Larry, you sure look a fool with those mis-matched socks!")

Off-Licence:
    Liquor Store

Off-Roader, 4x4:
    SUV (sport utility vehicle)

Operating Theatre:
    Operating Room

Overtake:
    Pass

-P-

Pants, Underwear (mens):
    Underwear, Briefs, Shorts

"Para's" (army):
    'Airborne'

Paraffin:
    Kerosene

Pavement:
    Sidewalk

Pay Packet:
    Pay Check

P.C. (Police Constable):
    Police Officer

P.E. Class:
    Gym, Phys. Ed.

Pelican, Zebra Crossings:
    Pedestrian Crossing

Pensioner, OAP:
    Senior Citizen

Petrol:
    Gas

Physiotherapist:
    Physical Therapist

Pillock, Pratt, Berk:
    Idiot, moron, etc.

Piles:
    Hemorrhoids

Pinch, Nick:
    Steal

'Piss-Off!:'
    'Get Lost!'

Pitch (sports):
    Field

Plait (hair):
   Braid.

Plaster:
   Band-Aid

P.M.T.:
    P.M.S. (pre-menstrual syndrome)

Pocket Money (child's):
   Allowance

Poof, Queer:
    Faggot, Fag, Queer, Homo

Poorly, Ill:
    Sick, Ill

Pop Socks:
    Knee-High's

Post:
    Mail
("I'll be back in a few minutes, I've got to fill the truck up with gas and mail a letter")

Post Code:
    Zip Code

Post-Mortem:
    Autopsy

Potholing, caving:
    Spelunking

Pram:
   Baby Carriage, Buggy

Prawn:
    Shrimp

Press-up:
    Push-Up

Primary School:
    Elementary School, Grade School

Propstand (push bike):
    Kickstand

Pub:
   Bar

Public School:
    Private School
(As it should be!)

Pudding, Sweets, Afters:
   Dessert

Puncture (tyre):
    Flat

Pushchair:
    Stroller

Push Bike, Bicycle:
    Pedal Bike, Bicycle

-Q-

Queue:
    Line, Line-up

Quid:
   Buck (slang for a dollar)