Starting in Oslo and ending in Stockholm

Not knowing what you are interested in (and its too early for skiing), consider Rick Steves' Scandinavian itinerary suggestions:
https://www.ricksteves.com/europe/scandinavia-itinerary

Scandinavia Itinerary By Rick Steves So much to see, so little time.
How to choose? To help you get started, I've listed my top picks for
where to go in Scandinavia, my plan for your best three-week trip, and
tips on when to go. Depending on the length of your trip, and taking
geographical proximity into account, here are my recommended
priorities: 4 days: Copenhagen, Stockholm (connected by a 5.5-hour
express train) 6 days, add: Oslo 8 days, add: Norway in a Nutshell
fjord trip, Bergen 10 days, add: Overnight cruise from Stockholm to
Helsinki 14 days, add: Ærø, Odense, Roskilde, Frederiksborg (all in
Denmark) 17 days, add: Aarhus (Denmark), Kalmar (Sweden) 21 days, add:
Tallinn (Estonia) and more time in capitals

https://store.ricksteves.com/shop/p/scandinavia-guidebook

Just sequence to match you start finish cities. Or cut down the number of places and add days to your points of interest. Europe Through the Backdoor has instructions on itinerary planning.
https://store.ricksteves.com/shop/p/europe-through-the-back-door

Day 1 - Oslo
Day 2 - Oslo/Holmenkollen/Frognerparken
Day 3 - Oslo/Bygdøy
Day 4 - Norway in a Nutshell trip to Bergen
Day 5 - Bergen
Day 6 - Fly to Copenhagen
Day 7 - Copenhagen
Day 8 - Train to Stockholm
Day 9 - Stockholm
Day 10 - Stockholm

I highly recommend you buy Rick Steves Scandinavia. In addition to itinerary tips, and the descriptions of places helping you to figure out what interests you, he has invaluable money saving tips for this expensive region.

I already purchased the book from Amazon - just been waiting for it to get delivered. For now, I wanted other people's suggestions who has gone. Sometimes i get better advice from these forums than the books. And RS books are the ONLY guide books I buy, take with me and rip them apart.

I think Nordheim's suggested itinerary is excellent. I would probably cut a day in Oslo (my least favorite Scandinavian city) and add it to Stockholm or Copenhagen. An extra day in Stockholm would give you time for a day trip. You can take overnight cruises to Helsinki or Tallinn and spend the day. The price is reasonable and you eliminate two nights of hotel expense. Both cities are worth seeing and easy to navigate on foot. An extra day in Copenhagen would give you time to see more in the city or take a day trip to Roskilde (great Viking ship museum). I will also note that keeping that day in Oslo gives you more time to get over jet lag if that is an issue for you.

Three countries, three crowns: Norwegian Krone, Danish Krone and Swedish Krona. Use the ATMs. Don't bother with buying Euros in the States.

I would say the itinerary looks very heavy on cities and light on countryside. If this suits you it's fine. Plus, Bygdoy in Oslo feels like the country if you are planning to spend the day there. The Viking Ship museum in Oslo is the best I've ever seen, better than Roskilde, so I would eliminate that. If you like dramatic natural scenery you should spend some time in fjord country in Norway -- it's absolutely spectacular. We rented a car for 3 days there driving out of Bergen and it was just wonderful. Returning the car to the airport was a breeze - you literally drive up, drop off and walk 2 minutes to the terminal. Easy airport connection from downtown Bergen on the Flybus, too - it only takes about 15 minutes. Bergen is just adorable and I wish we had more time there. Oslo can be seen in a day. We spent 3 days at the start of our trip in Copenhagen, then 2 days at the end. I LOVE Copenhagen and could have spent a month there easily. There are so many things to do both in the city and just outside the city it's hard to select an itinerary! Plus, Copenhagen is a great city for wandering, something we didn't have enough time for. But one thing in Copenhagen you must do is go to Torvehallerne market just steps away from the Norreport metro station. You will see locals living life to the fullest at this market which has two buildings with all sorts of foodie/drinkie stalls inside as well as outside in the lovely courtyard filled with produce stands and picnic tables. Buy what looks good, bring it to your table and enjoy some serious people watching. You will want to become Danish!

Thanks for the tips! :)

I would suggest buying a Lonely Planet guide, which is much more thorough than RS's book. I can only speak about Sweden. Stockholm is a great city, but think about other areas to visit on the way to Stockholm. On the way by train from copenhagen or Gotenburg, you could go to Eksjö (via Nessjö), Småland which has the most 17th century wooden buildings in Sweden…although recently had an unfortunate fire, but a wonderful town and area. Or go to Granna on the lake. Or take a boat through the Göta Canal on the way to Stockholm. Or head north to Kiruna in the Arctic Circle? While I like RS books for the logistics on the places he writes about, it is not thorough and is limited in scope (sorry, redundant). Sweden is a beautiful country and there is so much more to see than Stockholm. I suggest you limit your time to Norway and Sweden. Copenhagen can easily be picked up another time when visiting countries further south. Be sure to take an archipelago boat tour as well. Or if you read Pippi Longstocking books as a child, you might want to visit Vimmerby, Småland where Astrid Lindgren was from and where the stories are based. Read Steig Larsson's Millennium Series (Girl with the Dragon Tatoo, etc.) and stay on Sodermalm in Stockholm. There is really so much to chose from it would be a shame to just visit the large cities, and see the same type of things, IMO. Have a great trip!