Interestingly I was having the exact same problem on a Windows client running Eclipse Mars/Luna on hibernate 4.3 and 5.1 into and Oracle 11g DB via TNS (I used a full permutation of Eclipse/hibernate version as I noticed this was potentially an issued described online.)

In the end the problem still persists in that environment but when trying to replicate on my Mac locally I can generate POLO just fine against a local mysql db. Then when I try and target my remote Oracle DB I get a similar behaviors.

Almost seems as if I'm timing out. I also tried using ojdbc[5,6].jar to no avail. I'll try putting in a small local oracle instance and see if that changes the behavior. My two cents.....Please let me know if your interested in additional details as I still need to get the Windows/Mars/hib5.1/Oracle 11g scenario to work. Cheers..