The O'Neill Site, also known as 20CX18, is an archaeological site near the mouth of Inwood Creek about five kilometers south of Charlevoix, Michigan. It preserves evidence of Late Woodland occupation from roughly AD 1000 to 1455 and contains partially stratified layers that show repeated use over time. Excavations uncovered stone tools, pottery, and house remains, indicating that this was a settled place where people lived across generations rather than a temporary camp. Its importance was formally recognized when it was added to the National Register of Historic Places in 1971.