Peter Killian Gallagher (born August 19, 1955) is an American actor, musician and writer. Since 1980, Gallagher has played many roles in numerous Hollywood films. He starred as Sandy Cohen in the television drama series The O.C. from 2003 to 2007. He currently appears in a recurring role on the USA Network drama Covert Affairs as CIA Clandestine Services Director Arthur Campbell.
Gallagher was born in New York City, New York, the son of Mary, a bacteriologist, and Tom Gallagher, an advertising executive. He is of Irish Catholic background and was raised in Armonk, New York. Gallagher graduated from Tufts University, where he had been active in theatre, appearing in such shows as Stephen Sondheim's Company and singing with the all-male a cappella group the Beelzebubs.
Gallagher's debut movie performance was as one of the co-stars in the 1980 movie, The Idolmaker.