I found nothing in the product manual on IR code format. A search of the internet using Google indicates Onkyo uses NEC codes which would be compatible with the EZUIRT. HOWEVER, i suggest confirming that with the manufacturer or sales location. Not all information on the internet is accurate when not from the manufacturer which these hits were not.
I added the Onkyo TX-SR875 as a device to my Harmony remote. The EZUIRT recognizes the buttons assigned codes for this device. They are identified as NEC Ext by the SHN Utility when it reads the last IR code received by the EZUIRT. Not a guarantee the actual Onkyo remote would be recognized but I think it confirms the use of NEC codes.