Oceania is a premium line. I would choose an inside on O over anything on Celebrity, all day, every day.
Your price comparison is apples to oranges.
O ships are 1000 persons max. You are not inundated with photographers, sales of this and that. Specialty dining is included. There is no nickel and diming.
If you are looking for lower price, then you found it, but the two do not compare.