We will be on your ship. This is our second Viking Ocean and fourth Viking overall. Preparation is not too bad. Be sure you have whatever VISAs are required for Australian citizens (I suspect none unless you go off on your own in Russia... why would you?).
The biggest differences between Viking River and Viking Oceans are the number of passengers (and therefore no "evening meeting" for everyone in one space), MANY more dining options, some ports are closer into town as with river cruises -- and some NOT. Viking handles the customer service VERY well -- just as they do on river cruises. The Viking Daily and/or new shipboard app are vital to keeping yourself informed, and you'll want to get to know Explorer Desk staff (the people who answer questions and help fix excursions, etc.). As always, you get further being nice than crabby!
Since we will be the "guinea pigs" on the final portion of our itinerary (Japan to Vancouver), we should all be flexible about possible changes. Viking always learns from the first time they do something, and we are the people who will be discovering what they need to fix 🙂 We were on Viking Star in their first summer of ocean cruising and only the 3rd time they did the Baltic itinerary. We thought they handled the issues that came up pretty well. If you have to have everything P-E-R-F-E-C-T, you might want to add the silver spirits package so you can have a drink 24/7 (just kidding).
The cabin space on Oceans is a bit better, depending on your category, and the bathroom definitely bigger. There is a decent amount of drawer space in PV cabins. We are trying to figure out what LIGHT weight gadget we can take to maximize the vertical space for hanging things. It was fine for 15 days. We are facing climates with highs from 45 degrees F to 100 degrees F on the LONG cruise... therefore more clothing options needed. I plan to use a suitcase under the bed!
Book excursions and dining options as soon as you are allowed, even included ones. Unlike River cruises, excursions CAN get full. You need to reserve the time slot you want for included ones. Dining.... you can ALWAYS add more reservations when you walk on the ship. Just go to the Manfredi's or Chef's Table desk or call from your cabin any time. There is NO LIMIT to how many times you can to the special dining venues, just to how many you can reserve in advance. And the World Cafe is always wonderful. The grill by the pool offers another lunch time option. MANY more places to eat on board, each with its specialties and all included.
Final prep advice: take everything on Cruise Critic with a grain of salt. There is a definite element of panic-or-complaint present here, but MOST advice here is very, very helpful!