If you love the Nautica, Insignia or Regatta you will love the Sirena - it is the same ship (almost). I have done over 35 cruises and 10 on Oceania. I sailed on the Sirena when it was the R4 and when it was the Pacific Princess.
I love Oceania and loved our most recent cruise from Barcelona to Lisbon.
Some observations - Oceania billed this as a "renovated" ship" - I was not able to see much "renovation". New bed and new carpet - but that was about it.
Cabins are well organized - beds are fabulous!