BMI as with all airlines frequently oversell flights. However it rarely results in a situation where people need to be 'bumped' off a flight. In all my travels with BD ive only once saw them ask for volunteers to offload, and that was on a LHR-DUB flight. I have never seen it happen on MAN-ORD route.

If you have preassigned seats you can check by going to www.checkmytrip.com and with your reference number and surname it will bring up your booking. The seats will be shown, and you can also click on the seat no which will bring up a seat map. If you have not assigned seats already they will not show up on checkmytrip, so you will need to contact bmi reservations to get this done.