| #501 | Oil & petrochemical pollution. -- Barking : Elsevier, 1982-1985. -- ISSN 0143-7127 print. -- ISSN 1878-2035 online |
| #502 | Opera Corcontica : Krkonošské práce. -- Vrchlabí : Správa Krkonošského národního parku, 1964-. -- ISSN 0139-925X print. -- ISSN 1803-1412 online |
| #503 | Optics news. -- Washington : Optical Society of America, 1975-1989. -- ISSN 0098-907X |
| #504 | Optics & photonics news. -- Washington : Optical Society of America, 1990-. -- ISSN 1047-6938 |
| #505 | Organon. -- Warszawa : Instytut Historii Nauki Polskiej Akademii Nauk, 1964-. -- ISSN 0078-6500 |
| #506 | Österreichs Fischerei. -- : Osterreichischer Fischereiverband, 1948-. -- ISSN 0029-9987 |
| #507 | Oversigt over Selskabets Virksomhed ... -- Kobenhavn : Munksgaard, 1932-. -- ISSN 0368-7201 |
| #508 | Pacific science. -- Honolulu : University of Hawaii Press, 1947-. -- ISSN 0030-8870 print. -- ISSN 1534-6188 online |
| #509 | Pakistan journal of scientific and industrial research. -- Karachi : Pakistan Council of Scientific and Industrial Research, 1958-. -- ISSN 0030-9885 print. -- ISSN 2077-0863 online |
| #510 | Pakistan journal of scientific research. -- Lahore : Pakistan Association for the Advancement of Science, 1949-. -- ISSN 0552-9050 |