The question is, what is under /oracle as subdirectories ? Usually this type of behavior is seen when somebody makes a typo and say instead of putting a file under /oracle/data01 directory, they type, let's say /oracle/dat01. While /oracle/data01 is a filesystem and should not affect the size of /oracle, the latter, i.e., /oracle/dat01 gets created as a directory and whatever is put in there blows up the size of /oracle.
I bet you, a clever dba is performing a database dump and placing the ascii file unde oracle, either deliberately, assuming it has infinite space, or they made a typo and accidentally doing this. Talk to you dba's if you are not able to install the lsof tool. But in the long run, lsof is an invaluable tool that should be on every sysadmins list to install as an add-on.
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UNIX because I majored in cryptology...