The reason the homeowners exemption is different is because in 2005 the IL legistrature pass a 7% Rule, which basically said, in reference to the HOE, if a property sells the after 2005 the homeower will get the minimum tax break. (The maximum being 20,000 off the assessed value, the minimum being 5,000 off the assesed value which in 2008 equalled $460)
Since you bought your condo, Oak Park has been reassessed. There was a reassessment in 2005 and 2008. If your assessed value remained the same from 2004 to 2007 I can only guess you are in a building that either experiences very few sales, or there some type of freeze on the assessed value. Perhaps you bought from a senior citizen.
I agree with Alex, you should always appeal your taxes. The Oak Park Township Assessors Office is a great resource for helping understand your property taxes. You can reach them at 708.383.8005. Ask for Ali or Shelley.
Baird & Warner
805 W Lake St
Oak Park, IL 60301
For example my 1 bedroom condo on Harrison St. taxes went from:
2004- Building was converted from apartments to condos - $800/year
2005 - $850/year
2006 - $900/year
2007 - Cook Country reassessment year - $4,200/year
You are not alone in this situation. It is rather brutal and significantly increases your monthly payment for taxes. Keep making sure that your homeowners tax exemption is filed and always contest your taxes every year. It is one of the simplest things you can do, you can even do it online via the Cook County assessor's website.
Please let me know if this answers your question or need anything else!