As you know, SFO is a hub for United and Virgin America. United has by far the most (and often cheapest) flights from SFO to other hubs. Hence the odds are in favor of UA but of course, it could be Virgin, AA or DL - certainly not Southwest, Alaska.
Actually, we did a Barcelona to Lisbon Oceania cruise in late June and all four legs of our flight were United. I had my doubts, but they all took off and landed with a minute or two of the scheduled times. We also changed our return flights to add an extra day on the end and Oceania handled that well (for a fee!).
Airlines that Oceania Cruises uses for international trips
in Oceania Cruises
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Actually, we did a Barcelona to Lisbon Oceania cruise in late June and all four legs of our flight were United. I had my doubts, but they all took off and landed with a minute or two of the scheduled times. We also changed our return flights to add an extra day on the end and Oceania handled that well (for a fee!).