Oceana, formerly Ocean Princess, is the fourth sister to Princess' Cruises Sun Princess, Dawn Princess and Sea Princess. I have not sailed on a P&O ship but I have been on Sea Princess, and since she sails part of the year for P&O I can't imagine her being much different from when she sails for Princess. Which is a clumsy way of saying I would expect Oceana to be at least as nice as a Princess run ship.
As for the ship itself, many of us here at cruise-chat think the Sun Class ships are the ideal ship; large enough to have all the amenities you would expect from a luxury cruise ship, yet small enough to be more intimate and easy to get around.
I can't for the life of me imagine someone calling the ship "irritating". I would have to hear more to know how to even respond to a comment like that.
Personally, I love the Sun Class ships, and considering P&O's reputation and experience I would not hesitate to sail on any of their ships.