There are two types of JR West pass - Can i presume you have a Sanyo pass (4 day or 8day)?If so then the FASTEST way is to take the Haruka express to Nishikujo station (45mins) and then take a taxi - 1500yen(ish), 10mins. Or you could take a local train from Nishikujo to Osaka station (next to Hilton)
OR: take the slow JR train from KIX to Osaka station - 70mins
If you have a Sanyo pass then you can either sit in non-reserved seats on the Haruka or make a seat reservation for free - worth it.
Enjoy Osaka; have some takoyaki for me!