I have not tried ESL clss in CU or OU. But I agree with "Zhiqi". In order to be able to enroll university here, better way to do is to try TOEFL or CAEL. CAEL is faster but normally hard to pass. Usually people get 150 or 190. This is not too bad, you can take other courses at the same time as you take English. TOEFL is recgnized by all universities but it takes longer to get the result, and if you can not pass it at one time, you have to spend another couple of months to take another one.
I took CAEL, but I got only 150 (embarrassing, hehe).