Your train from Shin Osaka runs on the Tokaido Shinkansen　（platforms 14-19)
Your train to Shin Aomori runs on the Tohoku Shinkansen (platforms 20-23)

Now ... These two lines are managed by different companies, so you need to go through the transfer ticket gate. Follow the signs that indicate transfers to the Tohoku, Yamagata, Akita, Joetsu Shinkansen. It's a very quick transfer.

(If you do make a mistake and end up exiting the Shinkansen area just walk around to the Tohoku, Yamagata, Akita, Joetsu Shinkansen and enter there).