OMDoc stands for Open Mathematical Documents. It is a markup format for mathematical documents. While MathML only covers mathematical formulae and the related OpenMath standard only supports formulae and “content dictionaries” containing definitions of the symbols used in formulae, OMDoc covers the whole range of written mathematics.
OpenMath is a markup language for specifying the meaning of mathematical formulae for representing mathematical objects with their semantics, allowing them to be exchanged between computer programs, stored in databases, or published on the WWW. In contrast to MathML, which deals principally with the presentation of mathematical objects, OpenMath is solely concerned with their semantic meaning or content.