This orchestra rehearses in Naperville and is part of Bethany Lutheran's Bach Academy of Music. Not only do they have orchestras, but they also have an active chamber music program.
This orchestras is in Chicago and rehearses on Sat or Sun depending on which of the orchestras you are in. Auditions are very competitive and usually take place in April.
From what I hear this is a pretty good youth orchestra.
Their auditions are in June.
MYSO is located in the Southwest Suburbs at Lewis University. They rehearse on Wednesdays and their auditions are in August.
This orchestra rehearses on Monday nights in the Western Suburbs at North Central College. Auditions are in August.