If you happen to have a copy of the OED2 on CD, decompress.c may or may not help.

Usage:
{dump 0x0 0x4; dump 0x40 0x44} < OED2.DAT | hd
The first number is your start offset less 0x8000, the second your end offset.

dump 0x(first+8000) 0xsecond < OED2.DAT | decompress > OED2.sgml

The rest is all in dict with some scripting to take mkindex output and make it into an actual index.

Tested with sha1sum:
626fab18cc9a25feafcf4080901c834e3ca05af7	/n/oed/OED2.DAT