Question: How many USPS Regional Origin Facilities are located in Hawaii?
Answer: One. The only facility is located in Honolulu, on the island of Oahu.
Notes: Most USPS shipments that doesn't leave the state of Hawaii will need to be first processed in the Honolulu facility.
For example, if you mail a letter from Hilo (on the east side of the Big Island) to Kailua-Kona (on the west side of the Big Island), the USPS will first ship that letter to Oahu to process it, and then ship it back to the Big Island to deliver it.