The second most common area for the Pietras surname is Opole Lublin, and that area used to be in Russia, so he probably was not from the Opole/Oppeln
I only checked the LDS database quickly, and many Pietras listed there came from Germany. That's why I assumed Oppeln. But you're right, could as well be the other Opole. Thanks for pointing that out.
For the OP:
If you know the SSN of your ancestor (if he had one), you could try to find him in the Social Security Death Index and see where the SSN was originally issued. Then you could try to find his naturalization record in one of the courts nearby.