I have just returned from a cruise on oceanna and although very family friendly it does say in the P&O broture all babies have to be at least a year old to cruise.
P&O do provide a selection of baby foods onboard (however if you have a particular brand that you like to stick to it may be worth contacting P&O to check whether they would allow you to take your own onboard with you). If you don't mind, you will have to let P&O know anyway as they will have to ensure they have stock of their regular baby food.
We have been on a number of P&O cruises, you may like to take some jars and milk, I don't think they have onboard, or you could check with P&O. There is plenty of good food, fruit etc and I'm sure they will be accommodating to the needs of your baby. Have fun its a wonderful experience, we are going on sea princess, NCL and ocean village this year with our 7 year old, she loves cruising must like mummy and daddy.
A ship is like a hotel so take what you cant get in a hotel ie formula, nappies, jars of food - usually though if on semi solids you should be able to mash something up thats on buffett or menu but as I said it is a luxury hotel it just floats -.
It is along time since we cruised with a baby on P&O (10 years) but then they had a special services division whom you contacted before the cruise and you could give them an inventory of all your baby food requirements, including juice and baby formula (at that time it had to be available at Boots as that is who they purchased from). We did this and the day after we boarded we were showed where everything was packed into a box specifically for our baby and we were allocated a cupboard in the restaurant where we could leave the food and then remove or take to the cabin what we wanted. This was all included. I would be interested to know if they still do this - perhaps a lot of people dont know about it but its well worth phoning them to find ou.
I also seem to remember they can provide you with bottle sterilisers etc available from the Childrens Playroo.