O'Day is a surname of Irish origin. Notable people with the surname include:
Alan O'Day (1940–2013), American singer-songwriterAnita O'Day (1919–2006), American jazz singerAubrey O'Day (born 1984), American singer, dancer, actress, songwriter, fashion designer, former member of the group Danity KaneCaroline Love Goodwin O'Day (1875–1943), American politicianConstance O'Day-Flannery, an American author of romance novelsDaniel O'Day, one of northwestern Pennsylvania's earliest independent refiners to be brought into John D. Rockefeller's Standard Oil CompanyDarren O'Day (born 1982), Major League Baseball relief pitcher for the Baltimore OriolesDevon O'Day (born 1962), American radio personality, former plus-sized model for the Ford Modeling Agency, songwriter, and an authorGeorge O'Day (1923–1987), American sailor, Olympic champion and boat designerHank O'Day (1859–1935), American right-handed pitcher, umpire and manager in Major League BaseballJeremy O'Day (born 1974), offensive lineman for the Saskatchewan Roughriders of the Canadian Football LeagueJohn O'Day (1856-1933), American politicianMarcus O'Day (1897–1961), American physicistMolly O'Day (1911–1998), American film actressMolly O'Day (singer) (1923–1987), American country music vocalist in the late 1940sNell O'Day (1909–1989), accomplished equestrian and B-movie actress of the 1930s and 1940sPat O'Day (born 1934), Pacific Northwest broadcaster and promoter