We recieve an oceanic crossing clearance from Shanwick Oceanic Clearance Delivery but the eastern side of the North Atlantic (up to 30 West) is under the juristiction of the Shannon Oceanic Control Centre. These are not the same control entities.
The clearance contains the Aircraft Identification, the NAT Track the flight is cleared on, the mach number and the flight level to be maintained from the first reporting point.
So an example dialogue between an aircraft and Shanwick might be:
"Shanwick good morning Britannia Five Two Nine Bravo"
"Britannia Five Two Nine Bravo Shanwick Go Ahead"
"Britannia Five Two Nine Bravo is estimating 56 North 10 West at 1256, requesting NAT Track Charlie at Flight Level 340, maximum flight level 360"
"Britannia Five Two Nine Bravo, standby"
---(usually a five minute gap!)---
"Britannia Five Two Nine Bravo your clearance"
"Five Two Nine Bravo go ahead"
"Britannia Five Two Nine Bravo is cleared to Orlando via track charlie. From 56 North 10 West maintain Flight level 350 mach decimal 80."
Then we acknowledge and continue the flight.
Also at some point we have to give them a NAT track message ident to ensure that the NAT track message (printout) we picked up at the crew room before we set off is valid and correct.
Also important to note that the Flying Pilot will take the radios whilst the Non-Flying pilot obtains the crossing clearance, as you must remain in radio contact with the controlling authority (normally London, Shannon or Scottish) whilst picking up your clearance from Shanwick.
Hope this was of some interest.
I used to love the smell of Jet-A in the morning...