C624 looks fine, it is almost midships, close to elevators ( that's a plus) and above it is the Casino, not usually noisy, and below are passenger cabins.
A734- I would not want it. It is almost in the stern, so often mechanical noises, and there will be LOTS of walking to and fro to amenities.It is above cabins, that's good, but above it is a pool. If you do searches of reviews you will find numerous cruisers who complain of the noise of chairs being scraped basically across their roof, all day and often during the night for cleaning, no matter what the ship it's a problem .
Have a look at http://www.Beyondships.com you can take virtual tours, search deckplans etc.
I also use http://www.cruisedeckplans.com
A general rule is look for something midships on a fairly low deck. You are halfway between everything, elevators are close, and if the ocean swell bothers you, the motion will be felt less in that position than anywhere else.

Thanks for your reply, I cant see p&o oceana on cruisedeckplans.com, also wandered if you could put a link to the forums where you have seen negativity reguarding cabin on A deck, thanks again

You would have to do a search on Cruise Critic- I can't point to one cruise in particular, as I said,but over the years you will see repeated comments from cruisers who have stayed in cabins under pool decks and regretted it...

A734 looks to be below the spa and spa pool

Might not be to bad but is on the same side as the gym which can be an issue if open all hours.
Pool/hot tub machinery will be somewhere close.

very handy for this area and the aft bit of open deck can think of this as a balcony.