The dress code applies to the main dining room irrespective of what sort of dining you are taking.
You can wear a lounge suit if you want to on formal nights but the majority have a tux on these nights.
No only in the restaurants for selected dinning. usually twice a week is Formal Dress and the rest of the time usually smart casual. This is what we did on our cruise last year on the P&O Oceana but you can dine in self service restaurant and dress casual. Beryl Clayton. Birmingham.
P&o have suggested dress codes for certain evenings, ie smart casual, semi formal or formal, if you are eating in any of the restaurants they expect you to adhere to this, however on a formal night a dark suit is fine with a shirt & tie or kilts are allowed, but the majority of people do wear tuxedo's on formal nights, if you are just eating in the beachouse,waterside&just going in a bar smart casual is fine, but you may feel a little out of place on a formal night! hope this helps.
Sorry i was reading ventura reviews&didnt realise you were on about oceana, never sailed on oceana so cant comment.
The only place you do not have to adhere to the dress code of the evening is the buffet in the Plaza. In all the other restaurants and bars the dress code applies. A few men wear a lounge suit but I would say that over 95% wear DJs. Freedom dining is in one of the two main restaurants and the menu is exactly the same as the Club dining. Oceana is a lovely ship for the Caribbean, I'm sure you'll enjoy it. The ship takes on a lovely atmosphere on a formal dining night. So take that tuxedo and enjoy yourself.