Of course, that necessary information still needs to be stored somewhere......
I just noticed there are a total of 80 entries in oddtable.dat (counting the non-used entries marked "This is a non-used duplicate"). Hmm...there are also a total of 80 levels in the 10 level00X.dat files. I wonder......
So I opened up the level00X.dat files and compare, and sure enough, there is a one-to-one correspondence between the entries in oddtable.dat and level00X.dat.
For example, in level000.dat, the levels are:
"This should be a doodle!"
"What an AWESOME level"
"MENACING!!"
"One way digging to freedom"
"A BeastII of a level"
"All the 6`s........"
"Turn around young lemmings!"
"From The Boundary Line"
if you look at the first 8 entries of oddtable.dat, you'll see the entries are
"This is a non-used duplicate"
"This is a non-used duplicate"
"This is a non-used duplicate"
"This is a non-used duplicate"
"This is a non-used duplicate"
"This is a non-used duplicate"
"Not as complicated as it looks"
"Patience"
Notice how the non-used duplicates corresponds to the first 6 levels in level000.dat which are never repeated. The 7th entry in oddtable.dat is a repeat of the 7th level in level000.dat, and the 8th entry in oddtable.dat is a repeat of the 8th level in level000.dat.
You'll find something similar as you continue on. For example, the next 8 entries (entries 9th-16th) in oddtable.dat has a correspondence with the 8 levels in level001.dat, then the next 8 oddtable entries correspond to the 8 levels in level002.dat, etc.
(And in case you're wondering how "We all fall down" are repeated 4 times, well, it turns out there are 2 copies of the level in the level00X.dat files: one in level001.dat, and another in level006.dat.)
So it looks like oddtable.dat is better named duplicates.dat. It looks like for each of the 80 levels in the various level00X.dat files, you can specify a duplicate version via the corresponding entry in oddtable.dat.
Then somewhere else, unfortunately not in oddtable.dat, there is a master list of the 120 levels actually used. It's this master list that specifies where to find the level, and whether to use the "original" or "duplicate [oddtable]" version.