Documentation on the OTA API can be download from Quality Center.
Go to Help -> …
The file is called OTA_API_Reference.chm. It is a “help file” and once downloaded, you may need to unblock it. If you’re seeing this:
Right click on the file and select Properties. Then click “Unblock”
This file came from another computer and might be blocked to to help protect this computer.
Microsoft’s idea of security is to write insecure code and then instead of patching it, have you to click a button to perform normal actions that could just as easily still install a virus.
Fijiaaron 
Thanks for the info !
can you please post the advantages and disadvantages of OTA API and complete information of that if possible. because i am a beginner and would like to have a clear insight.
Advantages and disadvantages compared to what?
1. Updating Quality Center manually — it accomplishes a different goal. OTA is specifically for interacting with QC programmatically. If you don’t need to do that, it won’t help you. Whether you can accomplish the same thing through manual interaction easier is a choice you’ll have to make.
2. Directly updating data in the QC database — This may be possible, but I haven’t looked into it. It’s probably much more complex and there may be some obfuscation and security protections. If you know how to do it, it would probably be more performant, but there may also be logical constraints built into the application which could be bypassed with direct database access.
3. COM automation — I don’t even want to go down this route. Maybe you could write a script in QTP (or Selenium) to remote control a browser to interact with QC but I don’t want to think about it.
4. Some other method — I’ve heard that ALM 11 is supposed to include a web service interface. I don’t know anything about it or the extend to which it works. There are other third party tools that do use the OTA and you might choose to use them rather than using OTA directly.