Please don't report every "inconsistency" you come across immediately. Doing some research into why helps narrow down reports to the ones that actually *are* bugs. The best way to go about this is to assume that when you come across an odd behaviour, YOU are the one who is wrong until you can prove otherwise. Avoiding noise in the forum makes life easier for everybody, crying wolf everytime you see some random odd behaviour makes the real reports harder to follow.
The ctrl+o is placed for the exact same reason that the ctrl+k is reconstructed. If mIRC has the full line, it doesn't need to reconstruct the control codes. If only a partial copy is done within existing control codes, mIRC will insert ctrl+o at the end, that's just how it was decided to be implemented. If you have issue with the current behaviour, you should ask for a feature suggestion. The fact that mIRC inserts a ctrl+o is not a bug.
