All the downtown hotels with reasonable prices are full for my dates... I found quite a lot of good hotels near O'Hare airport, how convenient is access to downtown and how long would it take? Thanks a lot!
That may be a good idea if you don't mind a 45 min ride on the el/subway. Find a hotel close to either the Rosemont or Cumberland Ave station. Call the hotel to ask the Concierge the distance or if there is a shuttle service.
I used to take the blue line "L" from O'Hare to the Loop when I lived in the City. It took somewhere between 30 minutes and an hour to travel from O'Hare to Downtown Chicago, IIRC.
Look near the Cumberland or Rosemont station on the Blue Line. It'll take about 45-50 minutes for the train to reach downtown though. The Blue Line in this area runs in the median of the Kennedy Expressway.