THE bride will be on the right at this ceremony. Fox News contributor and Bill O’Reilly favorite Margaret Hoover – great-granddaughter of President Herbert Hoover – is engaged to marry John P. Avlon, a former Rudy Giuliani speechwriter and frequent New York Post op-ed contributor. Avlon, a centrist who wrote “Independent Nation,” proposed on bent knee in Robert F. Wagner Jr. Park in lower Manhattan.