>Sure, you can do it. Once you do, though, your code is not open source.

It is surely open source. It's just not Open Source. An important difference because of the capitalization.

If you don't want to deliberately confuse people, you would say "the source code is available" or something like that, because most people take open source to mean this:

http://opensource.org/osd

So that even if they're speaking out loud, where one can't see capitalization, everyone knows what they mean.