There is an interesting dynamic between the various lines.
I see some crossover from Seabourn to Oceania because Oceania has some great itineraries. However those that truly go on Seabourn for the service and cuisine more than the itinerary tend to go one time because there is a definite difference. Oceania owns that difference and balks if luxury is mentioned with it. (I am not so sure that will be the case when Marina is in full swing.)
As I don't do much with Regent I can't say if there is much cross-over with Oceania. I have not seen any evidence of it from reading across the internet or in speaking within the industry. I think the reason is pretty clear: Regent's hallmark is everything is included and if one finds value in that I strongly doubt Oceania's pay for most things approach would be attractive.
To me the fascinating possibility is a Crystal-Oceania or Crystal-Azamara cross-over. I just don't know enough to say if they exist, but I think that would be tailor-made.
What I do know is that Azamara-Oceania cross-overs are not significant...and that baffles me.
BTW, because there is a strong Celebrity-Azamara cross-over there was a change from Celebrity reps also handling Azamara...and then they were truly split. Too much crossover I guess.