The .ocdata file is created by the ownCloud server as a sanity-check on the data folder, to make sure that the folder it is looking at is really supposed to be ownCloud's data folder. The file has no contents (zero bytes in length), and all that it needs to do is exist and be detectable by the web server.
It is unclear if it needs to have any specific permissions or ownership. The parent folder (i.e. ownCloud's data folder) does need to give the web server read permission, but there will be other errors if that is not set up correctly.
There is a related error message: