What if I were to tell you that there are only four on this entire planet? Huh? Not counting Constable Odo, of course. Think of it - just four of us. And look at the havoc we've wrought.
~ "Miles O'Brien" stating the paranoia the four changelings have brought upon the Federation
This Miles O'Brien was a Changeling who was an agent of The Dominion in Star Trek: Deep Space Nine. Like the true Miles O'Brien, he was portrayed by Colm Meaney.

Brief History

He was supposedly one of four changelings that infiltrated Earth in the Earth year 2372. Their presence wasn't known until a bomb went off in Antwerp during an interstellar conference involving the Romulans and a few other galactic powers. It was the first terrorist attack of it's kind in a century on Earth. Because of their presence, which included this Miles', the Federation had become paranoid and forced it's citizens to go under blood screenings to confirm they are who they say they are. 

Later, Benjamin Sisko started to question the real reason why he was called back to Earth and was relieved of his post as acting chief of Security of Earth. Later on that night, this Miles would pay him a visit. He arrogantly told Benjamin that they would win because it's their fear that will be their undoing and that The Dominion doesn't fear them like the Federation fears them. After Benjamin asked in frustration if they are done with their conversation, Miles laughs and says "...they have barely begun." Of course he meant with the coming conflict against The Dominion and not the conversation.

As quicky as this Miles appeared to Benjamin, he left, leaving Benjamin to his thoughts on what his next move will be.