Oceana County is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 26,570. The county seat is Hart. Long known as part of the large Ojibwe territory, the county was founded by European Americans in 1840 and organized in 1855.Settlers were attracted by access along the White River, which reaches its mouth on Lake Michigan in Muskegon County to the south.