Miles O'Brien (Changeling)
The impostor Chief Miles O'Brien was a Changeling and Founder of the Dominion. In 2372, this Changeling temporarily replaced the Human Starfleet enlisted officer known by the same name, and was one of four that had infiltrated Earth. 

In this form in 2372, he startled Benjamin Sisko when he appeared to him, as Sisko sat alone on a bench at Starfleet Headquarters, during an intentional attempt to contact Sisko. During their talk, he explained to him their strategy of creating distrust among Humans and revealed that only four Changelings were at the time operating on Earth. He told him that if a few Changelings could cause so much panic, think what an invasion would do. ()

Changelings