| #831 | Obzory matematiky, fyziky a informatiky. -- Nitra : Protonit, 1995-. -- ISSN 1335-4981 |
| #832 | OECD. Economic outlook. -- Paris : OECD, 1967-. -- ISSN 0474-5574 print. -- ISSN 1609-7408 online |
| #833 | Operations research. -- Baltimore : The Operations Research Society of America, 1956-. -- ISSN 0030-364X print. -- ISSN 1526-5463 online |
| #834 | Operations research letters. -- Amsterdam : North-Holland, 1981-. -- ISSN 0167-6377 print. -- ISSN 1872-7468 online |
| #835 | Opsearch. -- Delhi : Operation Research society of India, 1964-. -- ISSN 0030-3887 print. -- ISSN 0975-0320 online |
| #836 | Optimization. -- Yverdon : Akademie-Verlag, 1977-. -- ISSN 0233-1934 |
| #837 | Optimization Letters. -- : Springer, 2007-. -- ISSN 1862-4472 print. -- ISSN 1862-4480 online |
| #838 | Order. -- Dodrecht : Reidel, 1984-. -- ISSN 0167-8094 print. -- ISSN 1572-9273 online |
| #839 | Osaka Journal of Mathematics. -- Osaka : Osaka University, Department of Mathematics, 1964-. -- ISSN 0030-6126 |
| #840 | Osaka mathematical journal. -- Osaka : Osaka University. Faculty of Science. Department of Mathematics, 1949-1963. -- ISSN 0388-0699 |