Ques. I am looking for a USB OTG device for my Samsung Galaxy S-II
Ans. Yes, you can attach USB ON The GO cable on Samsung galaxy S-II or any other Android Smart Phone, to attach external storage or any other usb device.
Now you have 2 options.
1) Buy simple USB OTG Cable and then attach any pendrive to it.( cost less than Rs. 50)
Buy this if you already have a pendrive and hassle free portability is not a priority.
2) Buy USB OTG Pendrive. This pendrive has both standard usb port as well as micro usb port on it. So, it connects to both PC and Smart Phones without the need of any cable.( cost Rs: 500-600)==>Sandisk Ultra Dual 16 GB On-The-Go Pendrive
Buy this if you want portability on your side and are looking for new pendrive.