| Oberlin Old Upground Reservoir Lorain County | 17 | 1960 | Low | Fair |
| Oberlin Upground Reservoir Lorain County | 20 | 1960 | High | Fair |
| Obringer Upground Reservoir Crawford County | 13 | 1990 | Significant | Fair |
| Ohio Power Company Pond Dam Mum-100 Muskingum County | 15 | — | Low | Fair |
| Ohio Power Company Pond Mb-115 Dam Morgan County | 38 | 1967 | Low | Fair |
| Ohio Power Company Pond Mb-141 Dam Morgan County | 38 | 1968 | Low | Fair |
| Ohio Power Company Pond Mb-144 Pond Morgan County | 30 | 1968 | Low | Fair |
| Ohio Power Company Pond Mb-46 Dam Morgan County | 59 | 1960 | Significant | Fair |
| Ohio Power Company Pond Mm-52 Dam Morgan County | 35 | 1962 | Significant | Fair |
| Ohio Power Company Pond Mmv-11 Dam Morgan County | 50 | 1964 | Significant | Fair |
| Ohio Power Company Pond Nb-43 Dam Noble County | 60 | 1966 | Significant | Fair |
| Ohio Power Company Pond Nb-64 Dam Noble County | 38 | 1962 | Low | Fair |
| Ohio Power Company Pond Nb-90 Dam Noble County | 51 | 1974 | Significant | Fair |
| Ohio River Collieries Sediment Pond Dam Guernsey County | 26 | 1979 | Low | Fair |
| Osu Golf Course Lake Dam Franklin County | 17 | 1936 | Significant | Fair |
| Ottawa Upground Reservoir Putnam County | 22 | 1972 | High | Fair |
| Owens Corning Science & Tech Lake Dam Licking County | 24 | 1958 | Significant | Fair |
| Pancake Lake Dam Summit County | 20 | 1965 | Low | Fair |
| Paradise Lakes - North Lake Dam Preble County | 32 | 1946 | High | Fair |
| Paulding Upground Reservoir Paulding County | 24 | 1966 | High | Fair |
| Paulmeadows Lake Dam Hamilton County | 32 | 1965 | Low | Fair |
| Perry County Recreation Assoc. Lake Dam Perry County | 19 | 1969 | Significant | Fair |
| Pheasant Run Lake Dam Lorain County | 11 | — | Significant | Fair |
| Pine Creek Structure No. 8 Dam Lawrence County | 36 | 1972 | High | Fair |
| Pine Hill Golf Club Lake No. 3 Dam Medina County | 56 | 1969 | Significant | Fair |