overnight bus trip from osaka to tokyo

i'll b arriving at kansai international airport at arnd 8pm, would i stl catch a overnight trip to tokyo? where and how? thanks

It is the most competitive overnight bus route in Japan with many companies operating even more services.

Willer is recommended for its cheap prices and low rates. Its last buses depart central Osaka around 22:30, which you should be able to catch with a 20:00 arrival at Kansai Airport if your incoming flight is on time:
http://willerexpress.com/

Other companies have even later departures. JR Bus has departures from central Osaka as late as 23:30:
http://www.jrbuskanto.co.jp/bus_route_e/cotimep01.cfm?pa=2&pb=1&pc=j00...
http://www.jrbuskanto.co.jp/bus_route_e/cotimep01.cfm?pa=2&pb=1&pc=j00...

I recommend to calculate about two hours from arrival at the airport until you arrive in central Osaka, although it depends a little where in Osaka you are going. Among the various bus terminals in central Osaka, Namba OCAT is particularly easily reached from the airport:
http://www.kate.co.jp/pc/e_time_table/e_ocat_tt.html

tnx uji,
can I buy bus ticket on d spot or needs reservation on-line?