Can any hobbyist recommend one or more good escort
agencies in Chicago, with a good selection of
providers, available on short notice for outcall
near the O'Hare airport hotels?

I frequently make short trips to Chicago, and
rarely have time to make plans ahead of time.

If they take credit cards, that is a plus.
Flat rate and web site with pics of providers is best.

(I found a good agency in Detroit, American
Beauties. Good selection, flat rate, 4 or 5 providers
on schedule most nights.)

If you must go with an agency Micheles and Secretly Yours are the most reputable in town. Search here and the other board, there's no shortage of posts about them both.  
I know Michelles doesn't take credit cards, not sure about SY's. I personally wouldn't use a CC in the hobby to save my life but to each their own.

www.michellesescorts.com
www.secretly-yours.com

try surethingescorts.com.  joe does a good job for his clients, and it's reflected in the girls.