What time are you planning to leave Tokyo and arrive at Shin Osaka station?

This will determine how long you have for sightseeing before your flight departs KIX.

Are there any particular sites you want to see in Osaka?

Your best option may be to head to Namba station on arrival into Shin Osaka station and store your luggage in a locker at the station.
Namba is close to Dotonbori.