| Optics Express | OPT EXPRESS | 1094-4087 |
| Optics Letters | OPT LETT | 0146-9592 |
| Optik | OPTIK | 0030-4026 |
| Optimal Control Applications and Methods | OPTIM CONTR APPL MET | 0143-2087 |
| Optimization | OPTIMIZATION | 0233-1934 |
| Optimization and Engineering | OPTIM ENG | 1389-4420 |
| Optimization Letters | OPTIM LETT | 1862-4472 |
| Optimization Methods and Software | OPTIM METHOD SOFTW | 1055-6788 |
| Optimum. Economic Studies | | 1506-7637 |
| Option/Bio | OPTION/BIO | 0992-5945 |
| Opto-electronics Review | OPTO-ELECTRON REV | 1230-3402 |
| Optoelectronics and Advanced Materials-Rapid Communications | OPTOELECTRON ADV MAT | 1842-6573 |
| Optoelectronics Instrumentation and Data Processing | OPTOELECTRONICS INSTRUMENTATION AND DATA PROCESSING | 8756-6990 |
| Optoelectronics Letters | OPTOELECTRONICS LETTERS | 1673-1905 |
| Optometry and Vision Science | OPTOMETRY VISION SCI | 1040-5488 |
| Optometry-journal of the American Optometric Association | OPTOMETRY | 1529-1839 |
| Opuholi Golovy i Sei | OPUHOLI GOLOVY I SEI | 2222-1468 |
| Opus | OPUS | 1517-7017 |
| Opuscula Atheniensia | | 0078-5520 |
| Opuscula Historiae Artium | OPUSCULA HISTORIAE ARTIUM | 1211-7390 |
| Opuscula Mathematica | | 1232-9274 |
| Opuscula Musealia | | 0239-9989 |
| Opuscula Philolichenum | OPUSCULA PHILOLICHENUM | 1941-7519 |
| Opuscula Romana. Annual of the Swedish Institute in Rome | | 0471-7309 |
| Opuscula-Annual of the Swedish Institutes at Athens and Rome | OPUSCULA-ANNUAL OF THE SWEDISH INSTITUTES AT ATHENS AND ROME | 2000-0898 |
| OR manager | OR MANAGER | 8756-8047 |
| OR Spektrum | OR SPECTRUM | 0171-6468 |
| Oral & Maxillofacial Pathology Journal | ORAL & MAXILLOFACIAL PATHOLOGY JOURNAL | 0976-1225 |
| ORAL and Implantology | ORAL AND IMPLANTOLOGY | 1974-5648 |
| Oral and Maxillofacial Surgery Cases | ORAL AND MAXILLOFACIAL SURGERY CASES | 2214-5419 |
| Oral and Maxillofacial Surgery Clinics of North America | ORAL MAXIL SURG CLIN | 1042-3699 |
| Oral and Maxillofacial Surgery-Heidelberg | ORAL AND MAXILLOFACIAL SURGERY-HEIDELBERG | 1865-1550 |
| Oral Diseases | ORAL DIS | 1354-523X |
| Oral Health & Preventive Dentistry | ORAL HLTH PREV DENT | 1602-1622 |
| Oral History Review | | 0094-0798 |
| Oral Microbiology and Immunology | ORAL MICROBIOL IMMUN | 0902-0055 |
| Oral Oncology | ORAL ONCOL | 1368-8375 |
| Oral Radiology | ORAL RADIOL | 0911-6028 |
| Oral Science International | ORAL SCIENCE INTERNATIONAL | 1348-8643 |
| Oral Surgery | ORAL SURGERY | 1752-2471 |
| Oral Surgery Oral Medicine Oral Pathology Oral Radiology | ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY ORAL RADIOLOGY | 2212-4403 |
| Oral Surgery Oral Medicine Oral Pathology Oral Radiology and Endodontics | | |
| Oral Surgery Oral Medicine Oral Pathology Oral Radiology and Endodontology | ORAL SURG ORAL MED O | 1079-2104 |
| Oral Therapeutics and Pharmacology | ORAL THERAPEUTICS AND PHARMACOLOGY | 0288-1012 |
| Oral Tradition | ORAL TRADITION | 0883-5365 |
| Oralprophylaxe und Kinderzahnheilkunde | ORALPROPHYLAXE UND KINDERZAHNHEILKUNDE | 1614-2217 |
| Orbis | ORBIS | 1856-1594 |
| Orbis Idearum | ORBIS IDEARUM | 2353-3900 |
| Orbis Linguarum | | 1426-7241 |
| Orbis Litterarum | | 0105-7510 |
