The Ohio River oil slick, so diluted after three weeks that it is now invisible, flowed past Cincinnati without any trouble. The 80-mile-long stretch of water containing the oil reached Cincinnati Sunday and was expected to completely pass by early today. City water officials had closed their Ohio River intake valves before the oil arrived. Because the city had a five-day reservoir of water, no conservation orders were issued. The slick, caused by the Jan. 2 collapse of an Ashland Oil Co. storage tank south of Pittsburgh, Pa., flowed past Cincinnati faster than it did many upstream cities because heavy rain and snowmelt have tripled the speed of the river.