As mler suggested, for post-1912 (?) naturalizations, the Oath of Allegiance was actually printed on the backside of the Petition for Naturalization form. It should be signed and dated and also include the number of the naturalization certificate which was assigned.
Copies of the certificate can be obtained only from the United States Citizenship & Immigration Service (USCIS). Since you (presumably) already have the actual certificate number, you can skip the index search and go directly to the "document request" procedure. Go to http://www.uscis.gov/portal/site/uscis/ ... 2ca60aRCRD and click "Order Online Now" at the top-right. On the next page, choose the third option, Record Request without Request Case ID.
After completeing the next few pages of information, you come to a page which looks like this. Check the first box and enter the letter "C" plus the certificate number in the box provided. Fill in the additional boxes in this section and then continue from there.

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