peter- arrange your flights however it suits you best. Yes you can do a one way in both directions and even on different airlines if you wish. The cruise line will not care how you get to the ship.
Typically a round trip airfare is less expensive than a one way, but if you can book it that way and save money, go for it.
Just be careful that you book travel for the correct date and give yourself time to get to and form the airport on both legs. We normally fly in a day before the cruise and spend the night in a hotel. Check a day or two in advance and after your cruise for prices too. Many times staying for an additional day can mean saving money on airfare too.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013