On average, listings lease in the $20's per square foot per year. The O'Fallon region is going through a a rise in rates. In the last 3 months, the median rate went up $22 per sqft per year.
In the last 90 days, 31 listings have come on the market, which means new listings most likley are going fast. You'll want to be prepared to make an offer immediately. About 19% of the available leases in the O'Fallon are subleases, so you might be able to find a shorter term or below market rent. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
Even though the median size is 22,780 sqft, there are still 0 office spaces below 2,000 sqft. Most buildings in this region are close to 2 stories. Most buildings in this area were built in 2001 and are Class A.