Check with your nationals to see if there is any policy regarding naming your classes by Greek Letter, name or symbol, etc. If you are a local, you can talk to past members or decide as a chapter to create your own unique system! As long as you aren't violating any laws or GLO policies, you're good to go
I would agree with 33girl-- if you are tracking your classes in Greek Letter order, then after Omega, you start again at Alpha Alpha.
* ADPi doesn't track new member classes by Greek Letter, but we will recall our new member classes as "Alpha Class of 2002, Alpha Class of 2003," etc.