What Roy says is the same as on the CIC website:
http://www.cic.gc.ca/EnGLish/information/security/police-cert/mid-east/oman.asp
" You will be required to submit the following documents at the time of application:
* a letter from your Omani sponsor (non-citizens only);
* a completed application form;
* a copy of your passport;
* two photographs (4x4 cm) with a blue background; and
* applicable fees."
So, in the event that you cannot get a letter from your Omani sponsor to obtain the PCC, you must write a letter to CIC submit with the application. In the letter, state all the pertinent details (dates when moved there/lived there, your address(es), your employer/sponsor), and explain why you cannot get the PCC (sponsor refuses to give you the letter). Describe all your efforts to obtain this, and the outcome of your efforts. This is what the CIC advised me when I called and asked what I should do if I could not get one from Cambodia (I haven't submitted yet, tho, so I don't know how they will respond).
I would also suggest including a statement whether you have had any arrests and/or convictions, and attach a copy of your residence permits/visas to prove you are there legally. If you have some kind of contract with your sponsor, include a copy of that also.
And hope for the best! Remember they also do their own security and background investigations, it doesn't end with the PCCs...