We have used Oceania Air for a number of cruises. Seats assigned have always been together.
My only issue is that some airlines allow you to change seats locations and others won't. Our TA is sometimes allowed and then another airline won't let her. Again depending on the airline we have been able to upgrade. Before deciding on air, I always check ITA Matrix for flights. Then working with our TA and ask for the flight we would like. Oceania occasionally refuses flights that was requested. We then look at what the alternative that was offered or do our own. Overall we have been satisfied with Oceania Air. The flights for us have been significantly cheaper.