I can't find the OS version used in iPad

Answer:
The iPad uses iPhone OS 3.2

There is beta information on Apple's developer pages.

From the iPad/iPhone Software Developer Kit page

“Start developing the next generation of innovative applications for iPad with iPhone SDK 3.2 beta. iPhone Developer Program members can download iPhone SDK 3.2 beta today.”

http://www.apple.com/ipad/sdk/

.

Answer:
It uses iPhone OS

Same as the iPhone and iPod Touch.