OMD is a searchable dictionary created by Dr Graham Dark and contains terms relating to biochemistry, cell biology, chemistry, medicine, molecular biology, physics, plant biology, radiobiology, science and technology. It includes: acronyms, jargon, theory, conventions, standards, institutions, projects, eponyms, history, in fact anything to do with medicine or science.

It aims to provide a one-stop source of information about all medical and scientific terms and includes many useful cross-references and pointers to related resources elsewhere on the Internet, as well as bibliographical reference to paper publications. It lacks many entries which one can find in paper dictionaries but contains more encyclopaedia-like entries and entries on various subjects. It also contains many definitions in related areas.

The dictionary started in early 1997 and has grown, to contain over 46,000 definitions totalling 17.5 megabytes. Entries are cross-referenced to each other and to related resources elsewhere on the net. It is freely available on the Internet via the World-Wide Web.

All searches are logged and a list of frequently requested missing terms is checked. Users are encouraged to contribute definitions of missing terms. These contributions are usually edited extensively before inclusion. New terms are added almost every day.

The dictionary is stored as a single source file in a simplified, easy-to-edit, human-readable form of mark-up which is converted to HTML on the fly by a Perl CGI script originally developed by Denis Howe at Imperial College for the Free On-Line Dictionary of Computing. The script uses Perl's extensive regular expression matching facilities to provide fast, indexed searches of headings as well as full-text searches. Other bits of the Perl script are used off-line to generate the lists of missing terms and the contents pages. It is hoped to develop this further to allow maintenance of the dictionary and associated files through a web form-based interface.