There is never a good time to go through Chicago. You have I-94, I-80, I-55, I-57 and I-65 all converge to dip under the lake. Combined that with millions of people commuting to and from work and it's a mess. Picture LA traffic on 4 lanes instead of 8 lanes and you get the idea. If you are delayed 2 hours in traffic, which is common, the 100 miles out of route is a deal.
Often hopped US-41 to avoid tolls in my semi. State even put a set of scales on US-41 so we would not feel lonely.
Personally from Oshkosh, I'd drop down to Madison on US-41, WI-26 and US-151 then take I-39 south to Bloomington IL, I-74 to Indy I-70 to Dayton. It's 568 miles vs 477 and no Chicago. 90 miles difference and no Chicago.... Depending upon traffic, it is probably quicker. Did I mention no Chicago?