To be added to OHG.:
Both dictionaries (the first of which is an old edition of what is now the OED) abbreviate not only Old High German in this way, but also Middle English (ME.), Old English (OE.), French (F.), etc. Ncik 23:20, 23 January 2006 (UTC)
The parentheses are not part of the abbreviation. But other ways of forming abbreviations (in the particular case of abbreviating language names in linguistic works, as well as in general) are possible: An Etymological Dictionary of the English Language, Rev. Walter W. Skeat, Oxofrd University Press, 1882 uses A.S., M.H.G., O.H.G., O.E., and the, to my eyes strange looking, O. Low G.. The last sentence and the following sentence violate the orthographical convention of omitting the period concluding an abbreviation if followed by a colon or full stop to avoid any ambiguity. A Dictionary of Slang and Unconventional English, Eric Partridge, London, George Routledge & Sons, Limited, 1937 uses M.E. and O.E.. Citing from the entry "period (full stop) in abbreviations" from A Dictionary of Modern English Usage, H. W. Fowler, 2nd edition, Oxford University Press, 1965: "Abbreviations are puzzling, but to puzzle is not their purpose, and everything that helps the reader to guess their meaning is a gain. One such help is to let him know when the first and last letters of the abbreviation are also those of the full word, which can be done by not using the period, ........ As to abbreviations formed by combining the initial letters of two or more words, practice is not uniform, but the tendency is to omit the periods -OED, BBC, UNO, NATE, etc." We have at least the following abbreviations for "Old High German": OHG, O.H.G., O. H. G., OHG. Ncik 16:37, 24 January 2006 (UTC)