To make my previous post more clear...
Three short forms of the Fraternity name are somewhat commonly used: APO, APhiO and APhiQ/APhiQue. The APhiQ/APhiQue form is generally used only at Historically Black Colleges and Universities (which UIS is not). The other two are used by various chapters across the country with APO being more common than APhiO. I *think* APhiO is used more commonly in Region V (West PA, Ohio, WV & East/Central Kentucky).
However *no* short form is official.
I've seen National Officers perfectly happy with brothers using each of the three. The only time I've actually seen advice given to use APhiO was for an Interest group at a school with a well established Alpha Psi Omega (The Drama Honorary) that used APO on campus. However in many places with both, if there is a desire to use different short forms, Alpha Psi Omega will use APsiO or (*shudder*) AYO. Given that it doesn't appear that UIS has APsiO, I don't think that's an issue...
In regards to your local chapters (the ones you have to deal with most often), of the four active chapters in your section (47), three have their email that is listed on the National website as apo @ school_domain.edu, which means they probably use APO as their abbreviation on campus.
Historically I have run into chapters that were refered to as the 'Omega's and as the 'Alpha Phi's
In short, if any brother breathes fire over the issue, feel free to direct them to me, I have a firehose with a pumper truck that is feeding from Lake Superior.