How to make a link between Drupal and my ownCloud server? I want my documents uploaded from Drupal to be directly stored on my ownCloud server. Drupal server and ownCloud are both installed in the same machine.

Do you know how to do this type of action?
As a first step you would need to share users between owncloud and drupal.

I found this github project that appears to allow your owncloud installation to use the users from the drupal database.

Anything beyond this point requires some coding.

Assuming that the above worked and you have the user shared between the two platforms, then I expect you would need to write a drupal module with that used the owncloud API to submit a file from a drupal form. You would also need to use the owncloud api to return a list of files associated with a user.

This would be a very crude implementation, which bypassed all of drupal's file handling functionality, and relied on your own code for every interaction with owncloud (I can't find an existing module). A more advanced module would aim to use owncloud as a backend for the file system, in which case you might want to look at the patterns used in the Droogle or dropbox modules and adapt them to the owncloud api.

Whilst writing this answer I found what looks to be an interesting discussion from drupalcon Austin entitled Collaborative Spaces: Integrating Drupal with Google Drive, Dropbox, and more. Maybe that would be of help.