- William O’Brien State Park is a 1,520-acre (6.2 km2) state park of Minnesota, USA, along the St. Croix River. Its hiking trails traverse rolling glacial moraine, riparian zones, restored oak savanna, wooded areas and bogs. It is a popular place for birdwatching, picnics, camping, cross-country skiing, canoeing, fishing, and other typical outdoor recreational activities.