I`ve cruised with P & O and the drinks prices are reasonable. When you board the ship you`ll need to open a shipboard account. Drinks will be purchased using a cruise card and paid for at the end of the cruise by credit card, cash or cheque.
I don't know about Ocean Village 2, but have been on Ocean Village 1 and the prices are very similar to pubs etc. you can't take your own drink on board, you're bags are searched each time you re-embark. You can buy duty free on board, but if you want to drink it in your cabin you have to pay a surcharge.
If you go to the OV website - oceanvillageholidays.co.uk- open the "on the boat" page, select entertainment & bars, then select watering holes. Scroll down until you see onboard OV2 and click on the Casbah drink list. It will show all prices.
My dad compares them to pub prices which sums them up perfectly really.