The original OISC implementation did not supply any means by which to read an ASCII character. Hence, the author of OISC ROT13 made a slight alteration to allow this. It doesn't add any other instructions, or make any really big changes, it simply adds two more memory mapped IO addresses.
Also available, for reference, is a partial implementation of cat(1) by the same author.
There are two historical tidbits in comp.compilers from Nov 1988 about OISCs. The original message and the reply.