The first option would be air travel - there are several flights from Kansai Intl (KIX) to Sapporo (New Chitose - CTS). Flights depart at 16:45, 17:55, and 20:50. The price is 39,900 yen (US $335 at today's rate).
Another option is taking a ferry and train combination from Osaka to Hokkaido. Unfortunately, the ferry takes a lot of time - 19 hours one way, plus a couple of hours for the train. So flight seems to be the best option here.
More options and detailed flight schedule:
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