From Ottawa to USA, you need preclear US immigration and custom... so you need to be at ottawa airport very early so if you stuck at US immigration you have plenty of time to deal with the officer... Once you clear US custom you will be OK to enter US... you do not need to do it again in WAS...
If you are holding a chinese passport, it's mandatory to fill out the I-94 form, please ask from the airline staffs when you check in at the counter..
Have fun...