Write the Ombudsman's at the address provided in your denial letter. It takes about six weeks (or it did for me), but my case was reviewed, and I was conditionally approved, then approved after the interview.
My process:
1) Because my denial was based on incorrect information I called the ORD station, as directed in my denial letter. The officer agreed that my denial was not warranted based on those facts.
2) She spoke with her supervisor and called me back the next day. Supervisor told her that the denial was from central vetting and there is nothing they can do about it, but to write the Ombudsman.
3) Wrote a letter and included any documentation.
4) Was approved about six weeks later.
5) Anecdotal information from the two live people with whom I interacted is that central vetting will deny in any cases where they see a problem, even if it is not warranted.