Hi Melissa,
First I recommend deciding on whether you will fly or travel by rail from Osaka - Sapporo or take the train. There's something to say for both.
Osaka - Sapporo by rail. Its a long trip, a little over 10 hours of train travel. You can however start at 6 am and get on an early Shinkansen to Tokyo. Then use the Hokkaido Shinkansen from Tokyo - Hakodate and finally a ltd. express train to Sapporo. Its quite a nice trip with bullet train travel on most of the way and the train views from Hokkaido are specially nice. You could be there at around 4pm and still have time to find a good place for dinner. Also this option will be very budget friendly, especially if you consider the cost of the JR Pass for just 1 day. The full route is included.
Osaka - Sapporo by plane. This will be faster than train travel but will cost more and is not as relaxed. Its 1h travel to Osaka/Kansai airport, then at least 90 minutes waiting for your flight. About 90 minutes airtime, 30 minutes for luggage collection and getting on the train and an other 30 minutes for train travel to Sapporo. So about 5 hours all together.
The JR Pass is also valid on the other routes for your itinerary.
Hope this helps,
Daniel
