"Luckily, no. The thought of being locked in the one shape all the time. It's unnatural."
The impostor Chief Miles O'Brien was a Changeling and Founder of the Dominion. In 2372, this Changeling temporarily appeared in the form of the Human Starfleet enlisted officer known by the same name, and was one of four that had infiltrated Earth.
Chatting with Sisko
His unexpected appearance initially startled Benjamin Sisko, as Sisko sat alone on a bench at Starfleet Headquarters, during an intentional attempt by the Founders to contact Sisko.
During their talk, he explained to Sisko that their strategy was to create distrust among Humans, before he revealed that only four Changelings were at the time operating on Earth. He told him that if only a few Changelings could cause so much panic, think what an invasion would do. (DS9: "Paradise Lost")
Miles O'Brien was played by actor Colm Meaney.
His demeanor was described in the script notes as "jaunty" and with a "cocky smile".