I recently said the following in chat:
Do you mean 90 cents, or 9 cents?
Okay, you left out the zero so I wasn't sure.
Afterwards it occurred to me I could have replaced the comma with a semi-colon. This led to a discussion with @ArdaXi over what the correct punctuation mark to use in this situation is: a comma, semi-colon, or period. The uncertainty arises from the fact that okay is a pro-sentence.
What is the proper punctuation to use when joining a pro-sentence with an independent clause?