Eventually I did find the four-story structure that housed OilCo. I went to OilCo's suite and asked the receptionist for Carmen Natoris, the audit engagement manager. Carmen was a very capable and good-spirited professional. She did her job well without the life or death ethos that some in the audit division carried around like martyrs.
"Sorry to fly you out for just the day, but Carmen said you had to be the one signing off on the taxes."