Most listings are in the $20's per square foot per year. The Oak Brook region is going through a decline in rates. In the last 3 months, the median rate decreased $2 per sqft per year. That's a 6% decline in rent.
Compared to the rest of the Chicagoland market, the Oak Brook submarket has higher rates, and you'll have to negotiate above asking rent to be in this hot region.
In the last 90 days, 31 listings have been posted online, which means spaces most likley are leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 3% of the available leases in the Oak Brook are subleases, so you will probably have to focus your search on direct office leases. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 2,070 sqft. Most properties in this region are close to 3 floors. Most buildings in this market were built in 1979 and are Class B.