| O canto da Matematica Elementar | pt | Sites with math lessons |
| Ocean of knowledge | ru,en | Others links |
| Odense University | en | Math servers (ftp and gopher) |
| Ohio Council of Teachers of Mathematics | en | Teacher-oriented sites |
| Ohio Council of Teachers of Mathematics | en | Teachers' associations |
| Oingo:Matemáticas | sp | Guides and educational servers |
| Oingo:Matemáticas | sp | Search engines and links guides |
| Oklahoma State | en | Math servers (ftp and gopher) |
| Oklahoma Council of Teachers of Mathematics | en | Teacher-oriented sites |
| Oklahoma Council of Teachers of Mathematics | en | Teachers' associations |
| Olimpiada Iberoamericana de Matematica | pt | Olympiads, competitions... |
| Olimpiada Matematica Argentina | sp | Olympiads, competitions... |
| Olimpiada Matematica Espanola | sp | Olympiads, competitions... |
| Olimpiada Mineira de Matematica | pt | Olympiads, competitions... |
| Olimpiadas Colombianas de Matematicas | sp | Olympiads, competitions... |
| Olimpiadas Iberoamericanas de Matematica Universitarias | pt | Olympiads, competitions... |
| Olimpiadas Portuguesas de Matematica | pt | Olympiads, competitions... |
| Olimpiadi della Matematica | it | Olympiads, competitions... |
| Olimpíadas de Matemática | pt | Olympiads, competitions... |
| Olimpíadas Portuguesas de Matemática | pt | Olympiads, competitions... |
| Oltre il compasso | it | History of mathematics |
| Olympiad Math Madness | en | Olympiads, competitions... |
| Olympiades de mathématiques | fr | Olympiads, competitions... |
| On-line Handbook of Integer Sequences | en | Math servers (ftp and gopher) |
| OnlineTesting | en | Sites with math tools |
| On-Math | en | Pedagogical journals |
| Ontario Association for Mathematics Education | en | Teacher-oriented sites |
| Ontario Association for Mathematics Education | en | Teachers' associations |
| Ontario Colleges Mathematics Association | en | Teacher-oriented sites |
| Ontario Colleges Mathematics Association | en | Teachers' associations |
| Open Directory - Science: Math | en | Sites with information on math-related Internet resources |
| Open Directory - World: Portugues: Ciencia: Matematica | pt | Guides and educational servers |
| Open Directory - World: Portugues: Ciencia: Matematica | pt | Search engines and links guides |
| OPTIMISATION LINEAIRE | fr | Methodical materials |
| Ordem de Contato | pt | Methodical materials |
| Order Course Outline | en | Education-related sites |
| Oregon Council of Teachers of Mathematics | en | Teacher-oriented sites |
| OSA Karlsruhe - Fortbildungsveranstaltungen | de | Texts and documents |
| otablo les maths en net! | fr | Sites offering various training techniques |
| Other Sources of Math Software Information | en | Links to math tools |
| Oxford English Dictionary | en | Encyclopaedia and musea |
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