See this from the P&O website
First of all, please do not worry about it. Who, when and how much you tip is at your discretion. Tips are usually given in cash at the end of your holiday to your Cabin Steward, Restaurant Stewards in the Club Dining restaurants, and to any other staff who you feel you wish to reward. The amount you tip is entirely up to you although, as a guide, we recommend a total amount per person (aged 12 and over) of £3.75 per day, which covers all staff who have looked after you during your holiday. The only exception to this is for passengers travelling on Oceana and Ventura who have opted to enjoy the flexibility of Freedom Dining as, with this style of dining, you may be seated at a different table and served by a different waiter on each occasion. We will therefore add an automatic service charge of £1.60 per passenger, per day to your shipboard account for the Restaurant Stewards. You will, of course, be able to increase or decrease this charge at your discretion by visiting Reception at any point during your cruise. You can also tip any other staff, including your Cabin Steward, as you feel appropriate. Further details of who and when you should tip can be found in your cabin.