The contents of this page are pretty much a copy/paste of a post ChaosBlackMagic's made on the subject.
This is the large OMF font. For each printable Extended ASCII character (that is, those characters c >= 0x20), this file contains 8 bytes containing an 8×8 bitmap defining the appearance of the character; each byte defines one row of a character. Character c starts at offset 8 * (c-32) in the file.
As an example, the character “A” has c = 65, so its appearance is described by bytes 264 to 271 of the file, which are, in hexadecimal and binary:
@264 0x78 01111000 @265 0xC4 11000100 @266 0xC4 11000100 @267 0xFC 11111100 @268 0xC4 11000100 @269 0xC4 11000100 @270 0xC4 11000100 @271 0x00 00000000
Note that the low-order bit is rendered rightmost.
This is the small OMF font. The format is exactly the same as that of GRAPHCHR.DAT, except that each character is represented by a 8×6 bitmap, so there are only 6 bytes per character.