Hi
Freedom dining is detailed in the brochure, but is not available on Oriana, only Ventura and Oceana.
Oriana dining is early/ late sitting in the Peninsular or Oriental restaurants and there are the select dining option (ie you pay a small cover charge) are available in the Oriana Rhodes and Terrace Grill. I am Fairly sure there is also a buffet option in the Conservatory as well throughout the day.

Went ahead and booked the cruise today for next Christmas.

Thanks for the advice. If our lads do not wish to eat with us in the restaurant for the evening, will there be anywhere else for them to eat?

Will they also be able to wear smart jeans for the evening?

Yes, there are other eating options (buffet style). Apart from the formal evenings smart jeans are absolutely fine. We had our 3 adult kids (17,22 & 25) with us recently on the Ventura and they had a ball. To be honest, they spent more time with us than we had anticipated (including every evening meal), which was great!

Get yourself a P&O cruise brochure in any Travel Agent, as this details everything about the ship, eating options and dress code etc.