Mr. O'Brien, believed to be 56, had been ill for nearly two months and was hospitalized at Paoli Memorial Hospital before being transferred to a rehabilitation center in West Chester. He seemed to be improving about a week ago, Jim Loftus, the station manager, said.

Mr. O'Brien had been at WOGL, a classic hits station specializing in music from the 1960s and 1970s, since 2002. Loftus described Mr. O'Brien as a musicologist who not only knew his play list but was well-versed in contemporary music.

"I never knew a guy who loved being on the air as much as he did," said Anne Gress, WOGL's program director. "There was such joy in his voice. He was put on this earth nothing other than to be on the air."

Mr. O'Brien's first stint in Philadelphia was at the top-40 WFIL, a home of rapid-fire disk jockeys, from 1976 to 1979.

A Midwesterner, he started his career in 1969 at KUDL in Kansas City, according to a biography from the Broadcast Pioneers of Philadelphia. A year later, he went to KTLK in Denver. In the next six years, his radio station stops included WQXI in Atlanta, WCFL in Chicago, WXLO in New York and WOKY in Milwaukee. He landed in Philadelphia at WFIL, toward the end of its "Famous 56" halcyon days.

In 1979, he moved to Los Angeles, where he worked first at KFI and then at KIIS before moving to KWK in St. Louis in 1985. He stayed for nine years, until his return to Denver at KZDG.

He returned to Philadelphia in 1996, when he joined WYXR (Star 104.5). In 2002, he joined WOGL, where he hosted the 3p.m.-to-7 p.m. weekday shift. He won several Achievement in Radio awards in Philadelphia.

"A lot of guys like me looked to guys like him to inspire their careers," said Loftus. "We are incredibly saddened. He was one of the greats."

He is survived by his mother. Funeral arrangements were pending, but the station said services probably would be Friday.