I took a look at the pricing and I believe you are looking at the pricing for one person for the Marina with air (double occupancy) and two people on the Seabourn Odyssey. (no air).
The price for the lowest available B3 Veranda stateroom on Oceania for two is about $8,300 vs. the $12,000 or so on the Seabourn Odyssey for a V3 Suite. That is $800 per day for two vs. $860 per day on Seabourn.
When you add gratuities of about $12.50 x 2 x 10 days ($250) and then your drinks, it looks like Marina is actually a more expensive cruise.
Now, that said, if you want 10 different restaurants rather than 4 and you don't care if there are 450 or 1,250 people things may not seem so skewed one way or the other.
What I looked at immediately is that the itineraries are sooooo different. So are your more concerned about the ship or the itinerary. All the money in the world for an itinerary you really don't want is a bad investment IMHO.