I-15 enters Oneida County from Box Elder County, Utah, south of the hamlet of Woodruff. The four-lane Interstate heads north parallel to the Union Pacific Railroad's Malad Subdivision rail line, a road named Old Highway 191, and the Malad River along the eastern edge of the wide Malad Valley adjacent to the Malad Range. I-15 has a diamond interchange with Woodruff Road at Woodruff and follows the western edge of Caribou National Forest. The highway passes just to the east of the county seat, Malad City, where the highway has a diamond interchange with the eastern end of Idaho State Highway 38 (SH 38). I-15 continues along Deep Creek to another diamond interchange with the western end of SH 36. From there, the Interstate follows the narrow valley of Devil Creek between the Bannock Range to the east and Elkhorn Mountain to the west. I-15 has one more diamond interchange in Oneida County next to Devils Creek Reservoir before the highway reaches Malad Summit, where the highway leaves the Great Salt Lake watershed and enters the Snake River watershed.