I've sailed on all three. Oceania is a step below Regent in luxury - food is comparable in most cases between O and R but R can do more in the way of special requests. Service is wonderful on both. The Regent ships are all suite and mostly all balcony. Oceania's penthouse suites are comparable to all the lowest cabins. Both offer ships of 700 passengers approx. Regent is all inclusive of beverages and Oceania is like Celebrity - sign and order for drinks. Entertainment is better on Regent. I posted a detailed review in the Oceania forum here last year. We really like Oceania but if all were equal, we'd sail Regent as it's our favorite. Fares because it's not all inclusive on Oceania are slightly less per diem if you choose penthouse on O vs. G or H category on Regent.

Here is a link to the review I did last year:
http://www.luxurycruisetalk.com/ubb/ultimatebb.php?ubb=get_topic;f=17;t=000090

We have since done a 16 day Baltic Itinerary and the cruise was perfect.